Stereo imaging system

ABSTRACT

The invention relates to a stereo imaging unit or system that can have a suitable degree of parallax and a wide angle of view even when used with an image pickup device smaller than film. The stereo imaging unit comprises a first entrance window  5 L and a second entrance window  5 R juxtaposed in the left-and-right direction, a plurality of reflecting surfaces  21 L,  22 L for guiding a light beam from the first entrance window  5 L to a single image pickup device  4 , a plurality of reflecting surfaces  21 R,  22 R for guiding a light beam from the second entrance window  5 R to the image pickup device  4 , a first negative lens group  1 L for the light beam from the first entrance window  5 L, a first positive lens group  3  located at an image side thereof via the longest air spacing in the lens system, a second lens group  1 R for the light beam from the second entrance window  5 R, and a second positive lens group  3  located on an image side thereof via the longest air spacing in the lens system, and satisfies conditions (1), (2), (3) and (4) for defining the focal lengths of the negative and positive lens groups in terms of the focal length of the overall lens system.

This application claims benefit of Japanese Application Nos.2003-148700, 2003-148701 and 2003-148702 filed in Japan on May 27, 2003,the contents of which are incorporated by this reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to a stereo imaging unit orsystem, and more particularly to a compact stereo imaging unit or systemcapable of phototaking stereo images having a large angle of view in thehorizontal direction.

So far, a camera capable of real-time phototaking two images havingparallaxes juxtaposed in the left-and-right direction for the samesubject has been known in the art. Since an oblong image pickup plane isusually used for phototaking, it is general that two images are arrangedside by side in the left-and-right direction. However, when it isdesired to obtain horizontally long stereo images with such aphototaking arrangement, two horizontally long images must be arrangedin the horizontal direction, resulting in a useless space on the imagepickup plane. For instance, when two such images are simultaneouslypicked up by means of a single image pickup device for range finding orthe like, it is difficult to obtain detailed image information.

To solve such a problem, patent publication 1 comes up with a stereoimaging optical system for capturing stereo images on an oblong imagepickup plane in the vertical direction.

In this optical system, a film plane is vertically divided into upperand lower areas, as shown in FIGS. 1 and 2 of patent publication 1, andleft and right images having parallaxes are guided to the upper andlower areas of the film plane by ways of two reflecting surfaces foreach.

FIG. 7 of patent publication 1 also shows a small-format, wide-anglestereo imaging optical system designed as a retrofocus type by locatinga negative lens on the subject side.

Patent Publication 1

JP(A)8-171151

However, the phototaking optical system set forth in this publication 1is designed exclusively for photographic film purposes; when it is usedwith an image pickup device having a small image pickup plane, nosufficient angle of view is obtainable, neither is anyangle-of-incidence dependence specific to the image pickup deviceobtained.

When the optical path for the phototaking optical system is taken apart,the left and right optical systems are broken down into independentoptical systems, and so it is difficult to reduce the number of lenscomponents involved.

Besides, the phototaking optical system of patent publication 1 has aspecific construction wherein the image plane is positioned on theoperator side. For this reason, the size of the optical system remainslarge in the depth (incident) direction, as viewed overall.

With the phototaking optical system set forth in patent publication 1,it is difficult to reduce the number of lens components involved,because when an optical path is taken apart, the left and right imagingoptical systems are broken down into independent optical systems.

SUMMARY OF THE INVENTION

In view of such problems with the prior art as described above, it isone object of the invention to provide a stereo imaging unit comprisinga stereo imaging optical system that can afford a suitable parallax anda wide angle-of-view thereto even when used with a small-format imagepickup device.

It is another object of the invention to provide a stereo imaging unitcomprising a small-format stereo imaging optical system that has a largeangle of view in the horizontal (parallactic) direction with a reducednumber of components involved.

It is yet another object of the invention to provide a stereo imagingunit comprising a small-format stereo imaging optical system that has alarge angle of view in the horizontal (parallactic) direction.

It is a further object of the invention to provide a stereo imaging unitcomprising a stereo imaging optical system capable of making bettercorrection for aberrations as well as a stereo imaging unit comprising astereo imaging optical system capable of making efficient use of imageshaving parallaxes on an image pickup device.

According to the first aspect of the invention, the above objects areachievable by the provision of a stereo imaging unit comprising a singleimage pickup device and a stereo imaging optical system capable offorming on said single image pickup device at least two parallacticimages having mutual parallaxes, characterized in that said stereoimaging optical system comprises:

a first entrance window and a second entrance window that have entrancesurfaces located on a subject side and are juxtaposed in aleft-and-right direction,

a plurality of reflecting surfaces for guiding a light beam incident onsaid first entrance window to said single image pickup device,

a plurality of reflecting surfaces for guiding a light beam incident onsaid second entrance window to said single image pickup device,

a first negative lens group having negative refracting power on thelight beam incident on said first entrance window and a first positivelens group that is positioned on an image side of said first negativelens group via the longest air space in a lens system and has positiverefracting power, and

a second negative lens group having negative refracting power on thelight beam incident on said second entrance window and a second positivelens group that is positioned on an image side of said second negativelens group via the longest air space in the lens system and has positiverefracting power, with satisfaction of conditions (1), (2), (3) and (4):−10.0<f _(N1) /f _(1T)<−2.0  (1)−10.0<f _(N2) /f _(2T)<−2.0  (2)1.5<f _(P1) /f _(1T)<10  (3)1.5<f _(P2) /f _(2T)<10  (4)where f_(N1) is the focal length of said first negative lens group,f_(N2) is the focal length of said second negative lens group, f_(P1) isthe focal length of said first positive lens group, f_(P2) is the focallength of said second positive lens group, f_(1T) is the focal length ofthe stereo imaging optical system including said first negative lensgroup, and f_(2T) is the focal length of the stereo imaging opticalsystem including said second negative lens group.

The advantages of, and the requirements for, the first stereo imagingunit according to the invention are now explained.

Referring first to the term “parallactic direction” used herein beforegiving an explanation of the invention, that term means a direction ofconnecting the position of a center ray incident from the same subjecton the entrance surface of the first negative lens group with theposition of a center ray incident on the entrance surface of the secondnegative lens group. Usually, a horizontal (left-and-right) direction ischosen in the invention; however, that parallactic direction is notalways limited thereto, and so could be selected from any desiredvertical or oblique directions. For a parallactic image on the imagepickup device (an image-formation plane), a relative misalignmentdirection of the same subject on a plurality of parallax images isdefined as that parallax direction.

The term “single image pickup device” used herein means one that has notonly one single receiving plane but also a plurality of juxtaposedreceiving planes on the same substrate (of usually a semiconductormaterial).

Referring now to the stereo imaging optical system in the stereo imagingunit of the invention, light beams entering the negative lens groupslocated in association with the entrance windows on the left and rightsides (that, unless otherwise stated, will stand for those in thehorizontal direction) are reflected at the associated plurality ofreflecting surfaces on the way to the image pickup device, with imageshaving parallaxes (parallax images) guided onto the image pickup deviceby way of the image-formation lens groups.

If, in this case, the left and right optical systems is generallyconfigured as a retrofocus type with the negative lens groups equivalentto objective lens groups and the positive lens groups located on theimage plane side and having an image-formation action, it is thenpossible to achieve a wide angle-of-view arrangement.

Conditions (1), (2), (3) and (4) are provided to define the focal lengthof each negative lens group and the focal length of each positive lensgroup in terms of the focal length of the whole optical system so as toobtain parallax images having a suitable angle of view and a suitableparallax even when used with an image pickup device of small size.

As the lower limit of −10.0 to conditions (1) and (2) is not reached orthe refracting power of each negative lens group becomes weak, anydesired wide angle of view cannot be obtained with an increase in thediameter of the negative lens group.

On the other hand, as the upper limit of −2.0 to conditions (1) and (2)is exceeded, the spacing between both the lens groups becomes narrow,rendering it difficult to bend an optical path in such a way as to havea suitable parallax.

Again, as the lower limit of 1.5 to conditions (3) and (4) is notreached or the refracting power of each positive lens group becomesweak, the spacing between both the lens groups becomes narrow, and asthe upper limit of 10 thereto is exceeded or the refracting power of thepositive lens group becomes small, the spacing between both the lensgroups becomes too wide, resulting in a bulky unit.

The lower limit to condition (1), and (2) should be set at preferably−8.0, and more preferably −6.0.

The upper limit to condition (1), and (2) should be set at preferably−3.0, and more preferably −4.0.

The lower limit to condition (3), and (4) should be set at preferably2.5, and more preferably 3.0.

The upper limit to condition (3), and (4) should be set at preferably7.0, and more preferably 5.0.

According to the second aspect of the invention, the first stereoimaging unit is further characterized by satisfying conditions (5) and(6):−0.4<β_(P1)<−0.06  (5)−0.4<β_(P2)<−0.06  (6)where β_(P1) is the transverse magnification of said first positive lensgroup, and β_(P2) is the transverse magnification of said secondpositive lens group.

The advantages of, and the requirements for, the second stereo imagingunit are now explained.

Conditions (5) and (6) are provided to define the transversemagnifications of the positive lens groups. As the lower limit of −0.4to condition (5), and (6) is not reached, there is an increased backfocus, leading to a bulky optical system. On the other hand, as theupper limit of −0.06 to condition (5), and (6) is exceeded, anywide-angle arrangement is unachievable and the diameter of the negativelens group becomes too large, because the refracting power of thenegative lens group becomes weak.

The lower limit to condition (5), and (6) should be set at preferably−0.3, and more preferably −0.25.

The upper limit to condition (5), and (6) should be set at preferably−0.1, and more preferably −0.15.

Enumerated below are the values of conditions (1) to (6) in the stereoimaging optical systems in the example to be given later.

f_(N1)=−22.908

f_(N2)=−22.908

f_(P1)=18.714

f_(P2)=18.714

f_(1T)=5.00

f_(2T)=5.00

f_(N1)/f_(1T)=−4.57

f_(N2)/f_(2T)=−4.57

f_(P1)/f_(1T)=3.74

f_(P2)/f_(2T)=3.74

β_(P1)=−0.218

β_(P2)=−0.218

According to the third aspect of the invention, the first or secondstereo imaging unit is further characterized in that a stop member toform an exit pupil is positioned in the spacing between said firstnegative lens group and said first positive lens group, and between saidsecond negative lens group and said second positive lens group.

The advantages of, and the requirements for, the third stereo imagingunit are now explained.

With this arrangement, it is easy to make a light beam incident on theimage pickup device telecentric, and it is possible to prevent thenegative and positive lens groups from having increased diameters.

According to the fourth aspect of the invention, the third stereoimaging unit is further characterized by satisfaction of conditions (7)and (8) while an optical path is taken apart:0.03<D _(PP1) /f _(P1)<1.5  (7)0.03<D _(PP2) /f _(P2)<1.5  (8)where D_(PP1) is the distance from said stop member to the entrancesurface of said first positive lens group, and D_(PP2) is the distancefrom said stop member to the entrance surface of said second positivelens group.

The requirements for the fourth stereo imaging unit are now explained.

As the lower limit of 0.3 to condition (7), and (8) is not reached,there is a tenuous effect on making the optical system telecentrictoward the image pickup device side. On the other hand, as the upperlimit of 1.5 is exceeded, off-axis light beams are likely to be shadedoff by the positive lens groups.

The lower limit to condition (7), and (8) should be set at preferably0.1, and more preferably 0.2.

The upper limit to condition (7), and (8) should be set at preferably1.0, and more preferably 0.5.

Set out below are the values of conditions (7) and (8) in the example tobe given later.

D_(PP1)=8.24

D_(PP2)=8.24

f_(P1)=18.714

f_(P2)=18.714

D_(PP1)/f_(P1)=0.440

D_(PP2)/f_(P2)=0.440

According to the fifth aspect of the invention, each of the 1^(st) to4^(th) stereo imaging units is further characterized in that while anoptical path entered from each entrance window is taken apart, eachoptical system is constructed as one having a substantially commonoptical axis, and a lens or a lens subgroup in at least a part of saidfirst positive lens group and said second positive lens group is a singelens or a lens subgroup that is located in front of said single imagepickup device and has a common optical axis.

The advantages of, and the requirements for, the fifth stereo imagingunit are now explained.

With this arrangement, the whole optical system can be designedsubstantially in the form of a coaxial one, and aberrations can beeasily corrected.

In addition, the number of components involved can be much more reducedbecause not only can the common image pickup device be used but also apart of the lens system can be used as a common member.

According to the sixth aspect of the invention, the 5^(th) stereoimaging unit is further characterized in that said plurality ofreflecting surfaces are arranged such that parallax images to beprojected on said single image pickup device are projected side by sidein a direction of juxtaposition different from that of said first andsecond entrance windows.

The advantages of, and the requirements for, the 6^(th) stereo imagingunit are now explained.

By arranging the plurality of reflecting surfaces in that direction ofjuxtaposition, light beams incident on the left and right entrancewindows form images on the image pickup device in a substantiallyvertical direction via the reflecting surfaces. Thus, if the imagepickup device is located in conformity to the contour of the juxtaposedleft and right parallax images, it is then possible to obtain imageshaving wide angles of view in the left-and-right direction.

In this connection, the term “different direction” encompasses verticaldirections as well as oblique directions, given that the parallaxdirection of the stereo imaging optical system is defined by theleft-and-right or horizontal direction.

According to the seventh aspect of the invention, the 6^(th) stereoimaging unit is further characterized in that the optical axis of saidfirst negative lens group the optical axis of said second negative lensgroup are unparallel with each other, and do not lie in the same plane.

The advantages of, and the requirements for, the 7^(th) stereo imagingunit are now explained.

When there is a large misalignment between the optical axes of thenegative lens group and the positive lens group with an optical pathtaken apart, it is likely to produce decentration aberrations withoutrecourse to correction of aberrations with both the negative andpositive lens groups.

When, as in the invention, the negative lens group and the positive lensgroup are substantially coaxial with an optical path taken apart,optical axis misalignments are of less significance, because residualaberrations symmetrical to the optical axis of the positive lens groupcan be corrected at the negative lens group that is substantiallycoaxial thereto.

However, when the optical axis of the negative lens group issubstantially in alignment with the optical axis of the positive lensgroup, it is impossible to carry out stereo imaging for a plurality ofparallactic images by simple bending of the optical axes on the sameplane, because of no overlap of left and right incident light beams onthe subject side.

Therefore, if both the negative lens groups are positioned such thattheir optical axes are unparallel with each other and do not lie in thesame plane as described above, it is then possible to separately guidethe left and right parallactic images having mutual parallaxes onto theimage pickup device by way of light beams from the common subject.

According to the eighth aspect of the invention, the 7^(th) stereoimaging unit is further characterized in that, given that the firstchief ray is defined by the center ray of a light beam that reaches thecenter of a parallactic image projected onto said single image pickupdevice via said first negative lens group, said plurality of reflectingsurfaces and said first positive lens group and the second chief ray isdefined by the center ray of a light beam that reaches the center of aparallactic image projected onto said single image pickup device viasaid second negative lens group, said plurality of reflecting surfacesand said second positive lens group, an angle difference between saidfirst chief ray incident on said first negative lens group and saidsecond chief ray incident on said second negative lens group is smallerthan an angle difference between the optical axis of said first negativelens group and the optical axis of said second negative lens group.

The advantages of, and the requirements for, the 8^(th) stereo imagingunit are now explained.

With such angle differences that satisfy the above requirement, thereare matching points in both the left and right parallactic images,which, for instance, can be utilized for measuring the distance of thesubject.

According to the ninth aspect of the invention, any one of the 6^(th) to8^(th) stereo imaging units is further characterized in that the imagepickup plane of said single image pickup device is configured in such arectangular shape as to have a long-side direction and a short-sidedirection, wherein the long-side direction of said image pickup plane isinclined with respect to the parallactic direction of said stereoimaging optical system.

According to the 10^(th) aspect of the invention, any one of the 6^(th)to 8^(th) stereo imaging units is further characterized in that saiddifferent direction is substantially orthogonal with respect to theparallactic direction of said parallactic images.

The advantages of, and the requirements for, the 9^(th) and 10^(th)imaging units are now explained.

With an optical system wherein, as contemplated herein, entrance-sidechief rays arriving at the respective parallactic images are mutuallyinclined while an optical path is taken apart, the image plane comes toincline with respect to the parallax direction of the stereo imagingoptical system. This is because, in order to bend the optical path suchthat both the chief rays (as defined in connection with the 8^(th)stereo imaging unit) come close to each other while the parallacticimages remain proximate to each other, at least one of the plurality ofreflecting surfaces must have its normal inclined with respect to aplane that includes the parallactic direction of the stereo imagingoptical system.

By inclining the image pickup plane of the image pickup device in such away as to lie along this inclining image plane, it is thus possible tomake effective use of the image pickup area. Especially by keeping thedirection of juxtaposition of the left and right parallactic imagessubstantially orthogonal to the parallactic direction of the parallacticimages, it is possible to the make the most of the image pickup area.

According to the 11^(th) aspect of the invention, any one of the 1^(st)to 10^(th) stereo imaging units is further characterized in that theimage pickup plane of said single image pickup device is configured insuch a rectangular shape as to have a long-side direction and ashort-side direction, and said single image pickup device is locatedsuch that a parallactic image by way of said first negative lens groupand a parallactic image by way of said second negative lens group areprojected side by side in the short-side direction of said single imagepickup device.

The advantages of, and the requirements for, the 11^(th) stereo imagingunit are now explained.

Since the left and right parallactic images are projected side by sidein the short-side direction of the rectangular image pickup device, itis possible to obtain more oblong paralactic images. It is in turnpossible to achieve a stereo imaging unit well fit for an onboard typestereo imaging gadget for which information having a wide angle of viewin a substantially horizontal direction is needed.

According to the 12^(th) aspect of the invention, any one of the 1^(st)to 11^(th) stereo imaging units is further characterized in that thedirection of scanning by said single image pickup device is inclinedwith respect to the parallactic direction of said stereo imaging opticalsystem.

According to the 13^(th) aspect of the invention, any one of the 1^(st)to 12^(th) stereo imaging units is further characterized in that thedirection of scanning by said single image pickup device issubstantially parallel with the parallactic direction of the parallacticimages.

The advantages of, and the requirements for, the 12^(th) and 13^(th)imaging units are now explained.

With an optical system wherein, as contemplated herein, entrance-sidechief rays arriving at the respective parallactic images are mutuallyinclined while an optical path is taken apart, the image plane comes toincline with respect to the parallax direction of the stereo imagingoptical system. This is because, in order to bend the optical path suchthat both the chief rays (as defined in connection with the 8^(th)stereo imaging unit) come close to each other while the parallacticimages remain proximate to each other, at least one of the plurality ofreflecting surfaces must have its normal inclined with respect to aplane that includes the parallactic direction of the stereo imagingoptical system. By inclining the direction of scanning by the imagepickup device in such a way as to lie along this titling image plane, itis thus possible to reduce image processing time.

According to the 14^(th) aspect of the invention, any one of the 1^(st)to 13^(th) stereo imaging units is further characterized in that saidsingle image pickup device is located such that the parallactic imageformed via said first negative lens group and the parallactic imageformed via said second negative lens group are projected side by side ina direction substantially orthogonal to the direction of scanning bysaid single image pickup device.

The advantages of, and the requirements for, the 14^(th) stereo imagingunit are now explained.

With the image pickup device designed to have a vertically divided,real-time readable image-receiving plane, image information processingtime can be shortened because real-time parallel processing can beimplemented without recourse to any memory.

According to the 15^(th) aspect of the invention, any one of the 1^(st)to 14^(th) stereo imaging units is further characterized by comprisingfield-limitation members for forming said at least two parallacticimages on the image pickup plane of said image pickup device in aseparate fashion.

The advantages of, and the requirements for, the 15^(th) stereo imagingunit are now explained.

Preferably, field-limitation members should be located in any desiredpositions in the left and right optical paths in such a way as to fendoff overlapping of the left and right parallactic images on the imagepickup plane. When, in this case, there are no intermediateimage-formation positions in the left and right optical paths, the fieldis limited by shading.

According to the 16^(th) aspect of the invention, the 15^(th) stereoimaging unit is further characterized in that at least one of saidfield-limitation members is said first entrance window and said secondentrance window, and a field mask having a substantially rectangularopening.

The advantages of, and the requirements for, the 16^(th) stereo imagingunit are now explained.

The field mask, if it is located at a position near to the subject forthe negative lens groups, makes it easy to implement a field-stopfunction. The field mask, if it is configured in the shape of the imageto be obtained (in a substantially rectangular shape), could have acombined field-stop function and hood function.

According to the 17^(th) aspect of the invention, the 16^(th) stereoimaging unit is further characterized in that said field mask is locatedat a position eccentric with respect to optical axes of said firstnegative lens group and said second negative lens group.

The advantages of, and the requirements for, the 17^(th) stereo imagingunit are now explained.

When it is desired to form parallactic images at any desired area of theimage pickup device, it is preferable to locate the field mask at aposition eccentric with respect to the negative lens groups regardlessof their inclinations.

According to the 18^(th) aspect of the invention, any one of the 1^(st)to 17^(th) stereo imaging units is further characterized in that, giventhat the first chief ray is defined by the center ray of a light beamthat reaches the center of a parallactic image projected onto saidsingle image pickup device via said first negative lens group, saidplurality of reflecting surfaces and said first positive lens group andthe second chief ray is defined by the center ray of a light beam thatreaches the center of a parallactic image projected onto said singleimage pickup device via said second negative lens group, said pluralityof reflecting surfaces and said second positive lens group, the contourof a lens in at least either one of said first negative lens group andsaid second negative lens group is in a non-rotationally symmetric shapethat comes closest to the optical axis of said lens on a side thereof,on which the associated chief ray is not incident.

The advantages of, and the requirements for, the 18^(th) stereo imagingunit are now explained.

Portions in the negative lens groups other than their effective surfacesshould be removed. Especially when the negative lens groups aredecentered with respect to the chief rays, some portions of the negativelens groups on their sides on which the chief rays are not incident areunnecessary. With the arrangement of the 18^(th) aspect of theinvention, size reductions can be achieved. This arrangement alsopermits the negative lens groups to function as a part of thefield-limitation members.

According to the 19^(th) aspect of the invention, there is provided astereo imaging system, characterized by comprising a stereo imaging unitas recited in any one of the 1^(st) to 18^(th) stereo imaging units, animage processor that is operable in response to an image from saidstereo imaging unit to calculate a subject distance, producing adistance signal and a controller that is operable in response to saiddistance signal to control other device.

According to the 20^(th) aspect of the invention, the 19^(th) stereoimaging unit is further characterized in that said other device is adisplay device.

According to the 21^(st) aspect of the invention, the 19^(th) stereoimaging unit is further characterized in that said other device is analarm device.

According to the 22^(nd) aspect of the invention, the 19^(th) stereoimaging unit is further characterized in that said other device is anoperating device.

According to the 23^(rd) aspect of the invention, there is provided astereo imaging unit comprising a single image pickup device and a stereoimaging optical system for forming at least two parallactic imageshaving mutual parallaxes on said single image pickup device,characterized in that said stereo imaging optical system comprises:

a first objective lens group having negative refracting power, and asecond objective lens group having negative refracting power and locatedwith a spacing provided therebetween,

an image-formation lens group having positive refracting power andlocated in an optical path on an image pickup device side with respectto said first objective lens group and said second objective lens group,

a 1-1^(st) reflecting surface for reflecting an incident light beam onsaid first objective lens group toward said first objective lens groupand a 1-2^(nd) reflecting surface for reflecting a light beam from said1-1^(st) reflecting surface toward said image pickup device, and

a 2-1^(st) reflecting surface for an incident light beam on said secondobjective lens group toward said first objective lens group and a2-2^(nd) reflecting surface for reflecting a light beam from said2-1^(st) reflecting surface toward said image pickup device, wherein:

said 1-2^(nd) reflecting surface and said 2-2^(nd) reflecting surfacesare located in such a way as to reflect light beams reflected thereattoward a subject, and said single image pickup device is located on aside of the light beams reflected at said 1-2^(nd) reflecting surfaceand said 2-2^(nd) reflecting surface.

The advantages of, and the requirements for, the 23^(rd) stereo imagingunit are now explained.

Referring first to the term “parallactic direction” used herein beforegiving an explanation of the invention, that term means a direction ofconnecting the position of a center ray incident from the same subjecton the entrance surface of the first objective lens group with theposition of a center ray incident on the entrance surface of the secondobjective lens group. Usually, a horizontal (left-and-right) directionis chosen in the invention; however, that parallactic direction is notalways limited thereto, and so could be selected from any desiredvertical or oblique directions. For a parallactic image on the imagepickup device (an image-formation plane), a relative misalignmentdirection of the same subject on a plurality of parallactic images isdefined as that parallactic direction.

The term “single image pickup device” used herein means one that has notonly one single receiving plane but also a plurality of juxtaposedreceiving planes on the same substrate (of usually a semiconductormaterial).

Referring now to the stereo imaging optical system in the stereo imagingunit of the invention, light beams incident on the juxtaposed (that,unless otherwise stated, will stand for the parallactic direction) leftand right negative objective lens groups (the first and second objectivelens groups) are reflected at the aforesaid respective reflectingsurfaces (the 1-1^(st) and 1-2^(nd) reflecting surfaces for the firstobjective lens group, and the 2-1^(st) and 2-2^(nd) reflecting surfacesfor the second objective lens group) on the way to the image pickupdevice, and images having parallaxes (parallactic images) are guidedonto the image pickup device by way of the image-formation lens group.

Thus, the image pickup device can be used as a common member, so thatthe number of components involved can be reduced, leading to reductionsin the size and weight of the stereo imaging unit.

When two such image pickup devices are provided for left and rightparallactic images, respectively, it is intractable to make correctionfor variations in the performance of the left and right image pickupdevices and read image information from the left and right image pickupdevices in synchronism. However, these problems can be solved by use ofthe common single image pickup device.

By imparting negative refracting power to the objective lens groups andpositive refracting power to the image-formation lens group on the imageplane side, a wide angle-of-view arrangement is achievable because theleft and right optical systems can each be designed as a retrofocustype.

In this regard, it is noted that when the size of the stereo imagingoptical system is further reduced by making its horizontal size small,any suitable parallax is not obtainable, encountering difficulty inobtaining effective stereo images. In the invention, the optical path isbent as described above so as to reduce increases in the size of thestereo imaging unit in its depth and height directions. To this end, theimage pickup device is interposed between the left and right imageobjective lens groups.

This enables the optical system to be much smaller than a conventionalarrangement wherein an optical path is bent in such a direction as toincrease in its depth and height direction size, because the imagepickup device is located in a dead space between the left and rightobjective lens groups.

According to the 24^(th) aspect of the invention, the 23^(rd) stereoimaging unit is further characterized in that said image-formation lensgroup is located just in front of said single image pickup device.

The advantages of, and the requirements for, the 24^(th) stereo imagingunit are now explained.

By locating the image-formation optical system just before the imagepickup device as defined above, it is unnecessary to make the diameterof the image-formation optical system larger than that of animage-formation optical system interposed only between two reflectingsurfaces, making easier it to afford telecentricity to the image pickupdevice.

According to the 25^(th) aspect of the invention, the 24^(th) stereoimaging unit is further characterized in that said image-formation lensgroup receives light beams for forming said at least two parallacticimages, and has only one optical axis.

The advantages of, and the requirements for, the 25^(th) stereo imagingunit are now explained.

In addition, the number of components involved can be much more reducedbecause not only can the common image pickup device be used but also thelens groups in the rear of the reflecting surfaces can be used as acommon member.

According to the 26^(th) aspect of the invention, any one of the 23^(rd)to 25^(th) stereo imaging units is further characterized in that said1-1^(st) reflecting surface, said 1-2^(nd) reflecting surface, said2-1^(st) reflecting surface and said 2-2^(nd) reflecting surface arearranged such that parallax images to be projected onto said singleimage pickup device are projected in a direction of juxtapositiondifferent from that of said first objective lens group and said secondobjective lens group.

The advantages of, and the requirements for, the 26^(th) stereo imagingunit are now explained.

By arranging the respective reflecting surfaces in that direction ofjuxtaposition, light beams incident on the left and right objective lensgroups form images on the image pickup device in a substantiallyvertical direction via the respective reflecting surfaces. Thus, if theimage pickup device is located in alignment to such image-formationpositions, it is then possible to obtain images at wide angles of viewin the left-and-right direction.

According to the 27^(th) aspect of the invention, the 26^(th) stereoimaging unit is further characterized in that, given that the firstvirtual optical axis is defined by the optical axis of saidimage-formation lens group as passing through said 1-2^(nd) reflectingsurface, said 1-1^(st) reflecting surface and said first objective lensgroup upon back ray tracing and the second virtual optical axis isdefined by the optical axis of said image-formation lens group aspassing through said 2-2^(nd) reflecting surface, said 2-1^(st)reflecting surface and said second objective lens group upon back raytracing, the first virtual optical axis entering said first objectivelens group and the second virtual optical axis entering said secondobjective lens group are unparallel with each other, and do not lie inthe same plane.

The advantages of, and the requirements for, the 27^(th) stereo imagingunit are now explained.

When the image-formation lens group acting as a common optical system tothe left and right objective lens groups is constructed such that itsoptical axis passes through a plurality of left and right reflectingsurface and objective lens groups upon back ray tracing, the left andright parallactic images are projected onto the image pickup device in apartly overlapping manner. To avoid this, it is necessary to shade offlight rays at the 1-2^(nd) and 2-2^(nd) reflecting surfaces that areoptical members nearest to the image-formation lens group side. Toprevent such shading, it is here assumed that the virtual optical axesare defined by optical axes that pass through the 1-2^(nd) and 2-2^(nd)reflecting surfaces upon enlargement and the objective lens groups.

By locating the virtual optical axes unparallel with each other andpermitting them not to lie in the same plane, it is thus possible toguide light beams from a common subject onto the common image pickupdevice by way of the image-formation lens group while they do notoverlap and, hence, to prevent any misalignment of the parallacticimages in terms of field. It is also possible to take a wide range ofdistance from the common subject to the stereo imaging unit.

According to the 28^(th) aspect of the invention, the 26^(th) stereoimaging unit is further characterized in that said first objective lensgroup and said second objective lens group are each comprised of a lensgroup with a rotationally symmetric optical axis, and with an opticalpath taken apart, each optical axis is substantially in alignment withthe optical axis of said image-formation lens group, and the opticalaxis of said first objective lens group and the optical axis of saidsecond objective lens group are unparallel with each other and do notlie in the same plane.

The advantages of, and the requirements for, the 28^(th) stereo imagingunit are now explained.

When there is a large misalignment between the optical axes of theobjective lens groups and the image-formation lens group with an opticalpath taken apart, it is likely to produce decentration aberrationswithout recourse to correction of aberrations with both the negative andpositive lens groups.

When, as in the invention, the objective lens groups and theimage-formation lens group are substantially coaxial with an opticalpath taken apart, optical axis misalignments are of less significance,because residual aberrations symmetrical to the optical axis of theimage-formation lens group can be corrected at the objective lens groupsthat are substantially coaxial thereto.

However, when the optical axes of the objective lens groups aresubstantially in alignment with the optical axis of the image-formationlens group, it is impossible to carry out stereo imaging for a pluralityof parallactic images having mutual parallaxes by simple bending of theoptical axes on the same plane, because of no overlap of left and rightincident light beams on the subject side.

Therefore, if both the objective lens groups are positioned such thattheir optical axes are unparallel with each other and do not lie in thesame plane as described above, it is then possible to separately guidethe left and right parallactic images having mutual parallaxes onto theimage pickup device by way of light beams from the common subject.

According to the 29^(th) aspect of the invention, the 27^(th) stereoimaging unit is further characterized in that, given that the firstchief ray is defined by the center ray of a light beam that reaches thecenter of a parallactic image projected onto said single image pickupdevice via said first objective lens group, said 1-1^(st) reflectingsurface, said 1-2^(nd) reflecting surface and said image-formation lensgroup and the second chief ray is defined by the center ray of a lightbeam that reaches the center of a parallactic image projected onto saidsingle image pickup device via said second objective lens group, said2-1^(st) reflecting surface, said 2-2^(nd) reflecting surface and saidimage-formation lens group, an angle difference between said first chiefray incident on said first objective lens group and said second chiefray incident on said second objective lens group is smaller than anangle difference between said first virtual optical axis entering saidfirst objective lens group and said second virtual optical axis enteringsaid second objective lens group.

According to the 30^(th) aspect of the invention, the 28^(th) stereoimaging unit is further characterized in that, given that the firstchief ray is defined by the center ray of a light beam that reaches thecenter of a parallactic image projected onto said single image pickupdevice via said first objective lens group, said 1-1^(st) reflectingsurface, said 1-2^(nd) reflecting surface and said image-formation lensgroup and the second chief ray is defined by the center ray of a lightbeam that reaches the center of a parallactic image projected onto saidsingle image pickup device via said second objective lens group, said2-1^(st) reflecting surface, said 2-2^(nd) reflecting surface and saidimage-formation lens group, an angle difference between said first chiefray incident on said first objective lens group and said second chiefray incident on said second objective lens group is smaller than anangle difference between the optical axis of said first objective lensgroup and the optical axis of said second objective lens group.

The advantages of, and the requirements for, the 29th^(th) and 30^(th)stereo imaging units are now explained.

With such angle differences that satisfy the above requirements, thereare matching points in both the left and right parallactic images,which, for instance, can be utilized for measuring the distance of thesubject.

According to the 31^(st) aspect of the invention, any one of the 26^(th)to 30^(th) stereo imaging units is further characterized in that theimage pickup plane of said single image pickup device is configured insuch a rectangular shape as to have a long-side direction and ashort-side direction, wherein the long-side direction of said imagepickup plane is inclined with respect to the parallactic direction ofsaid stereo imaging optical system.

According to the 32^(nd) aspect of the invention, any one of the 26^(th)to 31^(st) stereo imaging units is further characterized in that saiddifferent direction is substantially orthogonal with respect to theparallactic direction of said parallactic images.

The advantages of, and the requirements for, the 31^(st) and 32^(nd)imaging units are now explained.

With an optical system wherein, as contemplated herein, entrance-sidechief rays arriving at the respective parallactic images are mutuallyinclined while an optical path is taken apart, the image plane comes toincline with respect to the parallax direction of the stereo imagingoptical system. This is because, in order to bend the optical path suchthat both the chief rays come close to each other while the parallacticimages remain proximate to each other, at least one of the firstreflecting surface (the 1-1^(st) and 2-1^(st) reflecting surfaces) andthe second reflecting surface (the 1-2^(nd) and 2-2^(nd) reflectingsurfaces) must have its normal inclined with respect to a plane thatincludes the parallactic direction of the stereo imaging optical system.

By inclining the image pickup plane of the image pickup device in such away as to lie along this inclining image plane, it is thus possible tomake effective use of the image pickup area. Especially by keeping thedirection of juxtaposition of the left and right parallactic imagessubstantially orthogonal to the parallactic direction of the parallacticimages, it is possible to the make the most of the image pickup area.

According to the 33^(rd) aspect of the invention, any one of the 23^(rd)to 32^(nd) stereo imaging units is further characterized in that theimage pickup plane of said single image pickup device is configured insuch a rectangular shape as to have a long-side direction and ashort-side direction, and said single image pickup device is locatedsuch that a parallactic image by way of said first objective lens groupand a parallactic image by way of said second objective lens group areprojected side by side in a short-side direction of said single imagepickup device.

The advantages of, and the requirements for, the 33^(rd) stereo imagingunit are now explained.

Since the left and right parallactic images are projected in alignmentwith the short-side direction of the rectangular image pickup device, itis possible to obtain a more oblong paralactic image. It is in turnpossible to achieve a stereo imaging unit well fit for an onboard typestereo imaging gadget for which information having a wide angle of viewin a substantially horizontal direction is needed.

According to the 34^(th) aspect of the invention, any one of the 23^(rd)to 33^(rd) stereo imaging units is further characterized in that thedirection of scanning by said single image pickup device is inclinedwith respect to the parallactic direction of said stereo imaging opticalsystem.

According to the 35^(th) aspect of the invention, any one of the 23^(rd)to 34^(th) stereo imaging units is further characterized in that thedirection of scanning by said single image pickup device issubstantially parallel with the parallactic direction of the parallacticimages.

The advantages of, and the requirements for, the 34^(th) and 35^(th)imaging units are now explained.

With an optical system wherein, as contemplated herein, entrance-sidechief rays arriving at the respective parallactic images are mutuallyinclined while an optical path is taken apart, the image plane comes toincline with respect to the parallax direction of the stereo imagingoptical system. This is because, in order to bend the optical path suchthat both the chief rays come close to each other while the parallacticimages remain proximate to each other, at least one of the firstreflecting surface (the 1-1^(st) and 2-1^(st) reflecting surfaces) andthe second reflecting surface (the 1-2^(nd) and 2-2^(nd) must have itsnormal inclined with respect to a plane that includes the parallacticdirection of the stereo imaging optical system. By inclining thedirection of scanning by the image pickup device in such a way as to liealong this inclining image plane, it is thus possible to reduce imageprocessing time.

According to the 36^(th) aspect of the invention, any one of the 23^(rd)to 35^(th) stereo imaging units is further characterized in that saidsingle image pickup device is located such that the parallactic imageformed via said first objective lens group and the parallactic imageformed via said second objective lens group are projected side by sidein a direction substantially orthogonal to the direction of scanning bysaid single image pickup device.

The advantage of, and the requirements for, the 36^(th) stereo imagingunit is now explained.

With the image pickup device designed to have a vertically divided,real-time readable image-receiving plane, image information processingtime can be shortened because real-time parallel processing can beimplemented without recourse to any memory.

According to the 37^(th) aspect of the invention, any one of the 23^(rd)to 36^(th) stereo imaging units is further characterized by satisfyingconditions (11), (12), (13) and (14):−10.0<f _(T1) /f ₁<−2.0  (11)−10.0<f _(T2) /f ₂<−2.0  (12)1.5<f _(K) /f ₁<10  (13)1.5<f _(K) /f ₂<10  (14)where f_(T1) is the focal length of said first objective lens group,f_(T2) is the focal length of said second objective lens group, f_(K) isthe focal length of said image-formation lens group, f₁ is the focallength of the stereo imaging optical system including said firstobjective lens group, and f₂ is the focal length of the stereo imagingoptical system including said second objective lens group.

The advantages of, and the requirements for, the 37^(th) stereo imagingunits are now explained.

Conditions (11), (12), (13) and (14) are provided to define the focallength of each objective lens group and the focal length of theimage-formation lens group in terms of the focal length of the stereoimaging optical system so as to obtain images having a suitable angle ofview and a suitable parallax throughout the stereo imaging opticalsystem. As the lower limits of −10.0 to conditions (11) and (12) are notreached or the refracting power of each objective lens group becomesweak, any desired wide angle of view cannot be obtained with an increasein the diameter of the objective lens groups.

On the other hand, as the upper limit of −2.0 to conditions (11) and(12) os exceeded, the spacing between both the lens groups becomesnarrow, rendering it difficult to bend an optical path with a pluralityof mirrors interposed between them.

Again, as the lower limit of 1.5 to conditions (13) and (14) is notreached or the refracting power of the image-formation lens groupbecomes weak, the spacing between both the lens groups becomes narrow,and as the upper limit of 10 thereto is exceeded or the refracting powerof the image-formation lens group becomes small, the spacing betweenboth the lens groups becomes too wide, resulting in a bulky unit.

The lower limit to condition (11), and (12) should be set at preferably−8.0, and more preferably −6.0.

The upper limit to condition (11), and (12) should be set at preferably−3.0, and more preferably −4.0.

The lower limit to condition (13), and (14) should be set at preferably2.5, and more preferably 3.0.

The upper limit to condition (13), and (14) should be set at preferably7.0, and more preferably 5.0.

According to the 38^(th) aspect of the invention, any one of the 23^(rd)to 37^(th) stereo imaging units is further characterized by satisfactionof condition (15):−0.4<β_(K)<−0.06  (15)where β_(K) is the transverse magnification of said image-formation lensgroup.

The advantages of, and the requirements for, the 38^(th) stereo imagingunit are now explained.

Condition (15) is provided to define the transverse magnifications ofthe image-formation lens groups. As the lower limit of −0.4 to condition(15) is not reached, there is an increased back focus, leading to abulky optical system. On the other hand, as the upper limit of −0.06 tocondition (15) is exceeded, any wide angle-of-view arrangement isunachievable and the diameter of the negative lens group becomes toolarge, because the refracting power of the objective lens groups becomesweak. In addition, the diameter of the objective lens groups becomes toolarge.

The lower limit to condition (15) should be set at preferably −0.3, andmore preferably −0.25.

The upper limit to condition (15) should be set at preferably −0.1, andmore preferably −0.15.

Enumerated below are the values of conditions (11) to (15) in the stereoimaging optical systems in the examples to be given later.

f_(T1)=−22.908

f_(T2)=−22.908

f_(K)=18.714

f₁=5.00

f₂=5.00

f_(T1)/f₁=−4.57

f_(T2)/f₂=−4.57

f_(K)/f₁=3.74

f_(K)/f₂=3.74

β_(K)=−0.218

According to the 39^(th) aspect of the invention, any one of the 23^(rd)to 38^(th) stereo imaging units is further characterized by comprising astop member interposed between said first and second objective lensgroups and said image-formation lens group for forming an exit pupil.

The advantages of, and the requirements for, the 39^(th) stereo imagingunit are now explained.

With this arrangement, it is easy to make an incident light beam on theimage pickup device telecentric. It is also possible to keep theobjective lens groups and the image-formation lens group againstincreasing in diameter.

According to the 40^(th) aspect of the invention, the 39^(th) stereoimaging unit is further characterized by satisfying condition (16) whilean optical path is taken apart:0.03<D _(PK) /f _(K)<1.5  (16)where D_(PK) is the distance from said stop member to the entrancesurface of said image-formation lens group, and f_(K) is the focallength of said image-formation lens group.

The advantages of, and the requirements for, the 40^(th) stereo imagingunit are now explained.

As the lower limit of 0.03 to condition (16) is not reached, the effectof making the arrangement telecentric toward the image pickup deviceside becomes small. As the upper limit of 1.5 is exceeded, on the otherhand, an off-axis light beam is susceptible of shading by theimage-formation lens group.

The lower limit to condition (16) should be set at preferably 0.1, andmore preferably 0.2.

Regarding condition (16), the stereo imaging optical system in theexample to be given later has the following values.

D_(PX)=8.24

f_(K)=18.714

D_(PK)/f_(K)=0.440

According to the 41^(st) aspect of the invention, any one of the 23^(rd)to 40^(th) stereo imaging units is further characterized by comprisingfield-limitation members for forming said at least two parallacticimages on the image pickup plane of said image pickup device in aseparate fashion.

The advantages of, and the requirements for, the 41^(st) stereo imagingunit are now explained.

Preferably, the field-limitation members should be located in anydesired positions in the left and right optical paths in such a way asto fend off overlapping of the left and right parallactic images on theimage pickup plane. When, in this case, there are no intermediateimage-formation positions in the left and right optical paths, the fieldis limited by shading.

According to the 42^(nd) aspect of the invention, the 41^(st) stereoimaging unit is further characterized in that at least one of saidfield-limitation members is a field mask that is located on the subjectside of said objective lens groups and has a substantially rectangularopening.

The advantages of, and the requirements for, the 42^(nd) stereo imagingunit are now explained.

The field mask, if it is located at a position of the objective lensgroups near to the subject, makes it easy to implement a field-stopfunction. The field mask, if it is configured in the shape of the imageto be obtained (in a substantially rectangular shape), could have acombined field-stop function and hood function.

According to the 43^(rd) aspect of the invention, the 42^(nd) stereoimaging unit is further characterized in that said field mask is locatedat a position eccentric with respect to the optical axes of saidobjective lens groups.

The advantages of, and the requirements for, the 43^(rd) stereo imagingunit are now explained.

When it is desired to form parallactic images at any desired area of theimage pickup device, it is preferable to locate the field mask at aposition eccentric with respect to the objective lens groups regardlessof their inclinations.

According to the 44^(th) aspect of the invention, any one of the 23^(rd)to 43^(rd) stereo imaging units is further characterized in that, giventhat the first chief ray is defined by the center ray of a light beamthat reaches the center of a parallactic image projected onto saidsingle image pickup device via said first objective lens group, said1-1^(st) reflecting surface, said 1-2^(nd) reflecting surface and saidimage-formation lens group and the second chief ray is defined by thecenter ray of a light beam that reaches the center of a parallacticimage projected onto said single image pickup device via said secondobjective lens group, said 2-1^(st) reflecting surface, said 2-2^(nd)reflecting surface and said image-formation lens group, the contour of alens in at least either one of said first objective lens group and saidsecond objective lens group is in a non-rotationally symmetric shapethat comes closest to the optical axis of said lens on a side thereof,on which the associated chief ray is not incident.

The advantages of, and the requirements for, the 44^(th) stereo imagingunit are now explained.

Portions in the objective lens groups other than their effectivesurfaces should preferably be removed. Especially when the objectivelens groups are decentered with respect to the chief rays, some portionsof the objective lens groups on their sides on which the chief rays arenot incident are unnecessary. With the arrangement of the 44^(th) aspectof the invention, size reductions can be achieved. This arrangement alsopermits the objective lens groups to function as a part of thefield-limitation members.

According to the 45^(th) aspect of the invention, there is provided astereo imaging system comprising a stereo imaging unit as recited in anyone of the 23^(rd) to 44^(th) aspects of the invention, an imageprocessor that is operable in response to an image from said stereoimaging unit to calculate a subject distance, producing a distancesignal and a controller that is operable in response to said distancesignal to control other device.

According to the 46^(th) aspect of the invention, the 45^(th) stereoimaging unit is further characterized in that said other device is adisplay device.

According to the 47^(th) aspect of the invention, the 45^(th) stereoimaging unit is further characterized in that said other device is analarm device.

According to the 48^(th) aspect of the invention, the 45^(th) stereoimaging unit is further characterized in that said other device is anoperating device.

According to the 49^(th) aspect of the invention, there is provided astereo imaging unit comprising a single image pickup device and a stereoimaging optical system for forming at least two parallactic imageshaving mutual parallaxes on said single image pickup device,characterized in that said stereo imaging optical system comprises:

an image-formation lens group of positive refracting power, which islocated in front of said single image pickup device, receives lightbeams for forming said at least two parallactic images, and has only oneoptical axis,

a first objective lens group and a second objective lens, each of whichhas negative reflecting power and which have entrance surfaces facing asubject side and are juxtaposed with a spacing therebetween in aparallactic direction,

a first light-guide optical system that includes a 1-1^(st) reflectingsurface and a 1-2^(nd) reflecting surface for guiding a light beamincident from a subject thereon via said first objective lens group tosaid image-formation lens group, and

a second light-guide optical system that includes a 2-1^(st) reflectingsurface and a 2-2^(nd) reflecting surface for guiding a light beamincident from the subject thereon via said second objective lens groupto said image-formation lens group.

The advantages of, and the requirements for, the 49^(th) stereo imagingunit are now explained.

Referring first to the term “parallactic direction” used herein beforegiving an explanation of the invention, that term means a direction ofconnecting the position of a center ray incident from the same subjecton the entrance surface of the first objective lens group with theposition of a center ray incident on the entrance surface of the secondobjective lens group. Usually, a horizontal (left-and-right) directionis chosen in the invention; however, that parallactic direction is notalways limited thereto, and so could be selected from any desiredvertical or oblique directions. For a parallactic image on the imagepickup device (an image-formation plane), a relative misalignmentdirection of the same subject on a plurality of parallactic images isdefined as that parallactic direction.

The term “single image pickup device” used herein means one that has notonly one single receiving plane but also a plurality of juxtaposedreceiving planes on the same substrate (of usually a semiconductormaterial).

Referring now to the stereo imaging optical system in the stereo imagingunit of the invention, light beams incident on the juxtaposed (that,unless otherwise stated, will stand for the parallactic direction) leftand right negative objective lens groups (the first and second objectivelens groups) are reflected at the aforesaid respective reflectingsurfaces (the 1-1^(st) and 1-2nd reflecting surfaces for the firstobjective lens group, and the 2-1^(st) and 2-2^(nd) reflecting surfacesfor the second objective lens group) into the common image-formationlens group, and images having parallaxes (parallactic images) are guidedonto the image pickup device.

Thus, the image pickup device and a part of the lens systems can be usedas a common member, so that the number of parts involved can be reduced,leading to reductions in the size and weight of the stereo imaging unit.

When two such image pickup devices are provided for left and rightparallactic images, respectively, it is intractable to make correctionfor variations in the performance of the left and right image pickupdevices and read image information from the left and right image pickupdevices in synchronism. However, these problems can be solved by use ofthe common single image pickup device.

By imparting negative refracting power to the objective lens groups andpositive refracting power to the image-formation lens group on the imageplane side, a wide angle-of-view arrangement is achievable because theleft and right optical systems can each be designed as a retrofocustype.

It is noted that the number of parallactic images is not always limitedto two, and so three or more parallactic images could be guided onto thesingle image pickup device. In this case, sets of objective lens groupsand light-guide optical systems corresponding to that number arerequired.

According to the 50^(th) aspect of the invention, the 49^(th) stereoimaging unit is further characterized in that said stereo imagingoptical system is constructed such that said at least two parallacticimages having parallaxes, to be projected onto said single image pickupdevice, are projected side by side in a direction different from theparallactic direction thereof.

The advantages of, and the requirements for, the 50^(th) stereo imagingunit are now explained.

With the thus constructed stereo imaging unit, light beams incident onthe left and right objective lens groups form images generallyvertically on the image pickup device by way of the associatedreflecting surfaces. By locating the image pickup device in conformitywith the contours of the juxtaposed left and right parallactic image, itis thus possible to obtain images having wide angles of view in theleft-and-right direction.

In this connection, the term “direction different from the parallacticdirection” encompasses vertical directions as well as obliquedirections, given that the parallax direction of the stereo imagingoptical system is defined by the left-and-right.

According to the 51^(st) aspect of the invention, the 50^(th) stereoimaging unit is further characterized in that bending of an optical pathfrom said image-formation lens group by said first light-guide opticalsystem and said second light-guide optical system upon back ray tracingis effected by reflections at only four reflecting surfaces; said1-1^(st) reflecting surface, said 1-2^(nd) reflecting surface, said2-1^(st) reflecting surface and said 2-2^(nd) reflecting surface.

The advantages of, and the requirements for, the 51^(st) stereo imagingunit are now explained.

For the bending of the optical path by the image-formation lens groupupon back ray tracing (an optical path upon back ray tracing from theimage pickup device side to the objective lens group side), no relianceis on bending by a refracting prism in the light-guide optical systems(the first and second light-guide optical systems). It is thus possibleto prevent occurrence of chromatic aberrations by the bending of theoptical path.

According to the 52^(nd) aspect of the invention, any one of the49^(th), 50^(th) and 51^(st) stereo imaging units is furthercharacterized in that, given that the first chief ray is defined by thecenter ray of a light beam that reaches the center of a parallacticimage projected onto said single image pickup device via said firstobjective lens group, said first light-guide optical system and saidimage-formation lens group and the second chief ray is defined by thecenter ray of a light beam that reaches the center of a parallacticimage projected onto said single image pickup device via said secondobjective lens group, said second light-guide optical system and saidimage-formation lens group, said first objective lens group is operableas an optical system for polarizing said first chief ray, and saidsecond objective lens group is operable as an optical system forpolarizing said second chief ray.

The advantages of, and the requirements for, the 52^(nd) stereo imagingunit are now explained.

With the stereo imaging optical system in the 49^(th) stereo imagingunit, it is difficult to bend each chief ray (the first chief ray, thesecond chief ray) within the same paper plane, because the commonimage-formation lens group is used. For this reason, it is preferablethat each objective lens group has an additional function of bending(polarizing) the corresponding chief ray.

It is acceptable that each objective lens group has a common opticalaxis or the lenses in each lens group are free from any common axis ofsymmetry.

According to the 53^(rd) aspect of the invention, the 52^(nd) stereoimaging unit is further characterized in that, given that the firstvirtual optical axis is defined by the optical axis of saidimage-formation lens group as passing through said first light-guideoptical system and said first objective lens group upon back ray tracingand the second virtual optical axis is defined by the optical axis ofsaid image-formation lens group as passing through said secondlight-guide optical system and said second objective lens group uponback ray tracing, the first virtual optical axis entering said firstobjective lens group and the second virtual optical axis entering saidsecond objective lens group are unparallel with each other, and do notlie in the same plane.

The advantages of, and the requirements for, the 53^(rd) stereo imagingunit are now explained.

When the image-formation lens group acting as a common optical system tothe left and right is constructed such that its optical axis passesthrough the left and right light-guide optical systems and objectivelens groups upon back ray tracing, the left and right parallactic imagesare projected onto the image pickup device in a partly overlappingmanner. To avoid this, it is necessary to shade off light rays at, forinstance, the 1-2^(nd) and 2-2^(nd) reflecting surfaces in therespective light-guide optical systems, which are optical membersnearest to the image-formation lens group side. To prevent such shading,it is here assumed that the virtual optical axes are defined by opticalaxes that pass through the optical members in the respective light-guideoptical systems, which are located nearest to the image-formation lensgroup, upon enlargement, and the objective lens groups.

By locating the virtual optical axes unparallel with each other andpermitting them not to lie in the same plane, it is thus possible toguide light beams from a common subject onto the common image pickupdevice by way of the image-formation lens group while they do notoverlap and, hence, to prevent any misalignment of the parallacticimages in terms of field.

According to the 54^(th) aspect of the invention, the 52^(nd) stereoimaging unit is further characterized in that said first chief rayincident on said first objective lens group and said second chief rayincident on said second objective lens group lie in substantially thesame plane.

The advantages of, and the requirements for, the 54^(th) stereo imagingunit are now explained.

By permitting the chief rays incident on the respective objective lensgroups to lie in much the same plane, it is thus possible to guide lightbeams from a common subject onto the common image pickup device by wayof the common image-formation lens group while they do not overlap and,hence, to prevent any misalignment of the parallactic images in terms offield. It is also possible to take a wide range of distance from thecommon subject to the stereo imaging unit.

According to the 55^(th) aspect of the invention, any one of the 49^(th)to 54^(th) stereo imaging units is further characterized in that saidfirst objective lens group and said second objective lens group are eachcomprised of a lens group with a rotationally symmetric optical axis;the optical axis of said first objective lens group and the optical axisof said second objective lens group are unparallel with each other andlie at positions of rotational symmetry about the optical axis of saidimage-formation optical lens; and with an optical path taken apart, theoptical axis of each objective lens group is substantially in alignmentwith the optical axis of said image-formation lens group.

The advantages of, and the requirements for, the 55^(th) stereo imagingunit are now explained.

When there is a large misalignment between the optical axes of theobjective lens groups and the image-formation lens group with an opticalpath taken apart, it is likely to produce decentration aberrationswithout recourse to correction of aberrations with both the objectivelens groups and the image-formation lens group.

When, as in the invention, the objective lens groups and theimage-formation lens group are substantially coaxial with an opticalpath taken apart, on the other hand, optical axis misalignments are ofless significance, because residual aberrations symmetrical to theoptical axis of the image-formation lens group can be corrected at theobjective lens groups that are substantially coaxial thereto.

However, when the optical axes of the objective lens groups aresubstantially in alignment with the optical axis of the image-formationlens group, it is impossible to carry out stereo imaging for a pluralityof parallactic images having parallaxes by simple bending of the opticalaxes on the same plane, because of no overlap of left and right incidentlight beams on the subject side.

Therefore, if both the objective lens groups are positioned such thattheir optical axes are unparallel with each other and lie at thepositions of rotational symmetry as described above, it is then possibleto separately guide the left and right parallactic images having mutualparallaxes having mutual parallaxes onto the image pickup device by wayof light beams from the common subject.

According to the 56^(th) aspect of the invention, the 53^(rd) or 54^(th)stereo imaging unit is further characterized in that an angle differencebetween said first chief ray incident on said first objective lens groupand said second chief ray incident on said second objective lens groupis smaller than an angle difference between said first virtual opticalaxis entering said first objective lens group and said second virtualoptical axis entering said second objective lens group.

According to the 57^(th) aspect of the invention, the 55^(th) stereoimaging unit is further characterized in that an angle differencebetween the said first chief ray incident on said first objective lensgroup and said second chief ray incident on said second objective lensgroup is smaller than an angle difference between the optical axis ofsaid first objective lens group and the optical axis of said secondobjective lens group.

The advantages of, and the requirements for, the 56th^(th) and 57^(th)stereo imaging units are now explained.

With such angle differences that satisfy the above requirements, thereare matching points in both the left and right parallactic images,which, for instance, can be utilized for measuring the distance of thesubject.

According to the 58^(th) aspect of the invention, any one of the 50^(th)to 57^(th) stereo imaging units is further characterized in that theimage pickup plane of said single image pickup device is configured insuch a rectangular shape as to have a long-side direction and ashort-side direction, wherein the long-side direction of said imagepickup plane is inclined with respect to the parallactic direction ofsaid stereo imaging optical system.

According to the 59^(th) aspect of the invention, any one of the 50^(th)to 58^(th) stereo imaging units is further characterized in that saiddirection different from the direction of juxtaposition of parallacticimages on said single image pickup device is substantially orthogonal tothe parallactic direction of said parallactic images.

The advantages of, and the requirements for, the 58^(th) and 59^(th)imaging units are now explained.

With an optical system wherein, as contemplated herein, entrance-sidechief rays arriving at the respective parallactic images are mutuallyinclined while an optical path is taken apart, the image plane comes toincline with respect to the parallax direction of the stereo imagingoptical system. This is because, in order to bend the optical path suchthat both the chief rays come parallel to each other while theparallactic images remain proximate to each other, at least one of thefirst reflecting surface (the 1-1^(st) and 2-1^(st) reflecting surfaces)and the second reflecting surface (the 1-2^(nd) and 2-2^(nd) reflectingsurfaces) in the light-guide optical systems must have its normalinclined with respect to a plane that includes the parallactic directionof the stereo imaging optical system.

By inclining the image pickup plane of the image pickup device in such away as to lie along this inclining image plane, it is thus possible tomake effective use of the image pickup area. Especially by keeping thedirection of juxtaposition of the left and right parallactic imagessubstantially orthogonal to the parallactic direction of the parallacticimages, it is possible to the make the most of the image pickup area.

According to the 60^(th) aspect of the invention, any one of the 49^(th)to 59^(th) stereo imaging units is further characterized in that theimage pickup plane of said single image pickup device is configured insuch a rectangular shape as to have a long-side direction and ashort-side direction, and said single image pickup device is locatedsuch that a parallactic image by way of said first objective lens groupand a parallactic image by way of said second objective lens group areprojected side by side in the short-side direction of said single imagepickup device.

The advantages of, and the requirements for, the 60^(th) stereo imagingunit are now explained.

Since the left and right parallactic images are projected in alignmentwith the short-side direction of the rectangular image pickup device, itis possible to obtain a more oblong paralactic image. It is in turnpossible to achieve a stereo imaging unit well fit for an onboard typestereo imaging gadget for which information having a wide angle of viewin a substantially horizontal direction is needed.

According to the 61^(st) aspect of the invention, any one of the 50^(th)to 60^(th) stereo imaging units is further characterized in that thedirection of scanning by said single image pickup device is inclinedwith respect to the parallactic direction of said stereo imaging opticalsystem.

According to the 62^(nd) aspect of the invention, any one of the 50^(th)to 60^(th) stereo imaging units is further characterized in that thedirection of scanning by said single image pickup device issubstantially parallel with the parallactic direction of the parallacticimages.

The advantages of, and the requirements for, the 61^(st) and 62^(nd)imaging units are now explained.

With an optical system wherein, as contemplated herein, entrance-sidechief rays arriving at the respective parallactic images are mutuallyinclined while an optical path is taken apart, the image plane comes toincline with respect to the parallax direction of the stereo imagingoptical system. This is because, in order to bend the optical path suchthat both the chief rays come close to each other while the parallacticimages remain proximate to each other, at least one of the firstreflecting surface (the 1-1^(st) and 2-1^(st) reflecting surfaces) andthe second reflecting surface (the 1-2^(nd) and 2-2^(nd) reflectingsurfaces) in the light-guide optical systems must have its normalinclined with respect to a plane that includes the parallactic directionof the stereo imaging optical system. By inclining the direction ofscanning by the image pickup device in such a way as to lie along thisinclining image plane, it is thus possible to reduce image processingtime.

According to the 63^(rd) aspect of the invention, any one of the 49^(th)to 62^(nd) stereo imaging units is further characterized in that saidsingle image pickup device is located such that the parallactic imageformed via said first objective lens group and the parallactic imageformed via said second objective lens group are projected side by sidein a direction substantially orthogonal to the direction of scanning bysaid single image pickup device.

The advantage of, and the requirements for, the 63^(rd) stereo imagingunit is now explained.

With the image pickup device designed to have a vertically divided,real-time readable image-receiving plane, image information processingtime can be shortened because real-time parallel processing can beimplemented without recourse to any memory.

According to the 64^(th) aspect of the invention, any one of the 49^(th)to 63^(rd) stereo imaging units is further characterized by satisfyingconditions (11), (12), (13) and (14):−10.0<f _(T1) /f ₁<−2.0  (11)−10.0<f _(T2) /f ₂<−2.0  (12)1.5<f _(K) /f ₁<10  (13)1.5<f _(K) /f ₂<10  (14)where f_(T1) is the focal length of said first objective lens group,f_(T2) is the focal length of said second objective lens group, f_(K) isthe focal length of said image-formation lens group, f₁ is the focallength of the stereo imaging optical system including said firstobjective lens group, and f₂ is the focal length of the stereo imagingoptical system including said second objective lens group.

The advantages of, and the requirements for, the 64^(th) stereo imagingunits are now explained.

Conditions (11), (12), (13) and (14) are provided to define the focallength of each objective lens group and the focal length of theimage-formation lens group in terms of the focal length of the stereoimaging optical system so as to obtain images having a suitable angle ofview and a suitable parallax throughout the stereo imaging opticalsystem. As the lower limits of −10.0 to conditions (11) and (12) are notreached or the refracting power of each objective lens group becomesweak, any desired wide angle of view cannot be obtained with an increasein the diameter of the objective lens groups.

On the other hand, as the upper limits of −2.0 to conditions (11) and(12) are exceeded, the spacing between both the lens groups becomesnarrow, rendering it difficult to bend an optical path with a pluralityof mirrors interposed between them.

Again, as the lower limits of 1.5 to conditions (13) and (14) are notreached or the refracting power of the image-formation lens groupbecomes weak, the spacing between both the lens groups becomes narrow,and as the upper limits of 10 thereto are exceeded or the refractingpower of the image-formation lens group becomes small, the spacingbetween both the lens groups becomes too wide, resulting in a bulkyunit.

The lower limit to condition (11), and (12) should be set at preferably−8.0, and more preferably −6.0.

The upper limit to condition (11), and (12) should be set at preferably−3.0, and more preferably −4.0.

The lower limit to condition (13), and (14) should be set at preferably2.5, and more preferably 3.0.

The upper limit to condition (13), and (14) should be set at preferably7.0, and more preferably 5.0.

According to the 65^(th) aspect of the invention, any one of the 49^(th)to 64^(th) stereo imaging units is further characterized by satisfactionof condition (15):−0.4<β_(K)<−0.06  (15)where β_(K) is the transverse magnification of said image-formation lensgroup.

The advantages of, and the requirements for, the 65^(th) stereo imagingunit are now explained.

Condition (15) is provided to define the transverse magnifications ofthe image-formation lens groups. As the lower limit of −0.4 to condition(15) is not reached, there is an increased back focus, leading to abulky optical system. On the other hand, as the upper limit of −0.06 tocondition (15) is exceeded, any wide angle-of-view arrangement isunachievable and the diameter of the objective lens groups become toolarge, because the refracting power of the objective lens groups becomesweak. In addition, the diameter of the objective lens groups becomes toolarge.

The lower limit to condition (15) should be set at preferably −0.3, andmore preferably −0.25.

The upper limit to condition (15) should be set at preferably −0.1, andmore preferably −0.15.

Enumerated below are the values of conditions (11) to (15) in the stereoimaging optical systems in the example to be given later.

f_(T1)=−22.908

f_(T2)=−22.908

f_(K)=18.714

f₁=5.00

f₂=5.00

f_(T1)/f₁=−4.57

f_(T2)/f₂=−4.57

f_(K)/f₁=3.74

f_(K)/f₂=3.74

β_(K)=−0.218

According to the 66^(th) aspect of the invention, any one of the 49^(th)to 65^(th) stereo imaging units is further characterized by comprising astop member interposed between said first and second objective lensgroups and said image-formation lens group for forming an exit pupil.

The advantages of, and the requirements for, the 66^(th) stereo imagingunit are now explained.

With this arrangement, it is easy to make an incident light beam on theimage pickup device telecentric. It is also possible to keep theobjective lens groups and the image-formation lens group againstincreasing in diameter.

According to the 67^(th) aspect of the invention, the 66^(th) stereoimaging unit is further characterized by satisfying condition (16) whilean optical path is taken apart:0.03<D _(PK) /f _(K)<1.5  (16)where D_(PK) is the distance from said stop member to the entrancesurface of said image-formation lens group while an optical path istaken apart, and f_(K) is the focal length of said image-formation lensgroup.

The advantages of, and the requirements for, the 67^(th) stereo imagingunit are now explained.

As the lower limit of 0.03 to condition (16) is not reached, the effectof making the arrangement telecentric toward the image pickup deviceside becomes small. As the upper limit of 1.5 is exceeded, on the otherhand, an off-axis light beam is susceptible of shading by theimage-formation lens group.

The lower limit to condition (16) should be set at preferably 0.1, andmore preferably 0.2.

The upper limit to condition (16) should be set at preferably 1.0, andmore preferably 0.5.

Regarding condition (16), the stereo imaging optical system in theexample to be given later has the following values.

D_(PX)=8.24

f_(K)=18.714

D_(PK)/f_(K)=0.440

According to the 68^(th) aspect of the invention, any one of the 49^(th)to 67^(th) stereo imaging units is further characterized by comprisingfield-limitation members for forming said at least two parallacticimages on the image pickup plane of said image pickup device in aseparate fashion.

The advantages of, and the requirements for, the 68^(th) stereo imagingunit are now explained.

Preferably, the field-limitation members should be located in anydesired positions in the left and right optical paths in such a way asto fend off overlapping of the left and right parallactic images on theimage pickup plane. When, in this case, there are no intermediateimage-formation positions in the left and right optical paths, the fieldis limited by shading.

According to the 69^(th) aspect of the invention, the 68^(th) stereoimaging unit is further characterized in that at least one of saidfield-limitation members is a field mask that is located on the subjectside of said objective lens groups and has a substantially rectangularopening.

The advantages of, and the requirements for, the 69^(th) stereo imagingunit are now explained.

The field mask, if it is located at a position of the objective lensgroups near to the subject, makes it easy to implement a field-stopfunction. The field mask, if it is configured in the shape of the imageto be obtained (in a substantially rectangular shape), could have acombined field-stop function and hood function.

According to the 70^(th) aspect of the invention, the 69^(th) stereoimaging unit is further characterized in that said field mask is locatedat a position eccentric with respect to the optical axes of saidobjective lens groups.

The advantages of, and the requirements for, the 70^(th) stereo imagingunit are now explained.

When it is desired to form parallactic images at any desired area of theimage pickup device, it is preferable to locate the field mask at aposition eccentric with respect to the objective lens groups regardlessof their inclinations.

According to the 71^(st) aspect of the invention, any one of the 49^(th)to 70^(th) stereo imaging units is further characterized in that, giventhat the first chief ray is defined by the center ray of a light beamthat reaches the center of a parallactic image projected onto saidsingle image pickup device via said first objective lens group, saidfirst light-guide optical system and said image-formation lens group andthe second chief ray is defined by the center ray of a light beam thatreaches the center of a parallactic image projected onto said singleimage pickup device via said second objective lens group, said secondlight-guide optical system and said image-formation lens group, thecontour of a lens in at least either one of said first objective lensgroup and said second objective lens group is in a non-rotationallysymmetric shape that comes closest to the optical axis of said lensgroup on a side thereof, on which the associated chief ray is notincident.

The advantages of, and the requirements for, the 71^(st) stereo imagingunit are now explained.

Portions in the objective lens groups other than their effectivesurfaces should preferably be removed. Especially when the objectivelens groups are decentered with respect to the chief rays, some portionsof the objective lens groups on their sides on which the chief rays arenot incident are unnecessary. With the arrangement of the 44^(th) aspectof the invention, size reductions can be achieved. This arrangement alsopermits the objective lens groups to function as a part of thefield-limitation members.

According to the 72^(nd) aspect of the invention, there is provided astereo imaging system which comprises a stereo imaging unit as recitedin any one of the 49^(th) to 71^(st) aspects of the invention, an imageprocessor that is operable in response to an image from said stereoimaging unit to calculate a subject distance, producing a distancesignal and a controller that is operable in response to said distancesignal to control other device.

According to the 73^(rd) aspect of the invention, the 72^(nd) stereoimaging unit is further characterized in that said other device is adisplay device.

According to the 74^(th) aspect of the invention, the 72^(nd) stereoimaging unit is further characterized in that said other device is analarm device.

According to the 76^(th) aspect of the invention, the 72^(nd) stereoimaging unit is further characterized in that said other device is anoperating device.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts, which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) and 1(b) are illustrative in perspective of exemplaryconstructions of the stereo imaging unit according to one embodiment ofthe invention.

FIG. 2 is a taken-apart optical path diagram for the left optical systemin the embodiment of FIGS. 1( a) and 1(b).

FIG. 3 is a longitudinal aberration diagrams for the lens system in theembodiment of FIG. 2.

FIG. 4 is a transverse aberration diagrams for the lens system in theembodiment of FIG. 2.

FIG. 5 is a diagram upon projection onto an x-z plane of the leftoptical system in the stereo imaging unit according to the embodiment ofFIG. 2.

FIG. 6 is a diagram upon projection onto a y-z plane of the left opticalsystem in the stereo imaging unit according to the embodiment of FIG. 2.

FIG. 7 is illustrative in perspective of the left optical system in thestereo imaging unit according to the embodiment of FIG. 2.

FIG. 8 is illustrative of the relations in position of the left andright parallactic images projected onto the image pickup plane of theimage pickup device in the stereo imaging unit according to theembodiment of FIGS. 1( a) and 1(b).

FIG. 9 is a block diagram for a stereo imaging system to which thestereo imaging unit according to the embodiment of the invention isapplied.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The stereo imaging unit of the invention is now explained with referenceto some specific embodiments.

FIGS. 1( a) and 1(b) are illustrative in perspective of exemplaryconstructions of one embodiment of the stereo imaging unit according tothe invention. The constructions of FIGS. 1( a) and 1(b) are the samewith the exception of the shapes of field masks 5L and 5R; they will beexplained as the same stereo imaging unit, unless otherwise specified.

In what follows, “L” and “R” are suffixed to numerical references totell components or elements belonging to the left optical path fromthose belonging to the right optical path, unless otherwise stated.

The stereo imaging unit is built up of, corresponding to left and rightoptical paths, a left objective lens group 1L and a right objective lensgroup 1R that are a left negative lens group and a right negative lensgroup, respectively; a first reflecting surface 21L and a secondreflecting surface 22L for reflecting light incident from the objectivelens group 1L thereon in this order, and a first reflecting surface 21Rand a second reflecting surface 22R for reflecting light incident fromthe objective lens group 1R thereon in this order; an image-formationlens group 3 that receives light reflected at the left and right secondreflecting surfaces 22L and 22R and is a common positive lens group; anda common, single image pickup device 4 that is located on the imageplane of the image-formation lens group 3.

Here the direction of light reflected at the first reflecting surface21L, 21R and the second reflecting surface 22L, 22R is explained. As canbe seen from FIGS. 1( a) and 1(b), the left first reflecting surface 21Lbends an optical path for light incident from the left objective lensgroup 1L thereon at an angle of substantially 90° toward the rightobjective lens group 1R, and the second reflecting surface 22L bends thethus bent optical path at an angle of substantially 90° in a directionsubstantially parallel with an optical path that enters the leftobjective lens group 1L and in the opposite direction, entering light inthe common image-formation lens group 3. Likewise, the right firstreflecting surface 21R bends an optical path for light incident from theright objective lens group 1R thereon at an angle of substantially 90°toward the left objective lens group 1L, and the second reflectingsurface 22R bends the thus bent optical path at an angle of generally90° in a direction substantially parallel with an optical path thatenters the right objective lens group 1R and in the opposite direction,entering light in the common image-formation lens group 3.

Therefore in the stereo imaging unit, the common image-formation lensgroup 3 and the common image pickup device 4 can be interposed betweenthe left and right objective lens groups 1L and 1R. Then, theleft-and-right direction width is determined by a distance between theends of the left and right objective lens groups 1L and 1R (i.e., thebase line length plus the aperture of one objective lens group), thedepth-wise thickness with respect to a subject is determined by adistance between the front end surfaces of the objective lens groups 1L,1R and the rear end surface of light-guide optical systems 2L, 2R madeup of the first reflecting surfaces 21L, 21R and the second reflectingsurfaces 22L, 22R, and the height is substantially set at smaller thanthe aperture of the objective lens groups 1L, 1R)—this is becauseportions in the objective lens groups 1L, 1R other than their effectiveareas can be trimmed off. It is thus possible to obtain a compact stereoimaging unit.

A left parallactic image of a binocular parallactic image, formed on theimage pickup device 4 by light incident from the left objective lensgroup 1L on the image-formation lens group 3 by way of the firstreflecting surface 21L and then the second reflecting surface 22L, isprojected onto a lower half of a rectangular image pickup plane of theimage pickup device 4 in an inverted fashion, and a right parallacticimage of the binocular parallactic image, formed on the image pickupdevice 4 by light incident from the right objective lens group 1R on theimage-formation lens group 3 by way of the first reflecting surface 21Rand then the second reflecting surface 22R, is projected onto an upperhalf of the rectangular image pickup plane of the image pickup device 4in an inverted fashion.

Here the parallactic direction of the whole stereo imaging unit opticalsystem is now explained. Given that left and right chief rays areindicated at 10L and 10R, that parallactic direction is defined by thedirection of a straight line A–A′ that connects points of incidence ofthe left and right chief rays 10L, 10R on the entrance lens surfaces ofthe left and right objective lens groups 1L, 1R or the field masks 5L,5R, and the parallactic direction of the parallactic image projectedonto the image pickup device 4 is defined by the direction of a straightline B–B′ parallel with the rectangular sides of the image pickup device4. As can be seen from FIGS. 1( a) and 1(b), therefore, the parallacticdirection A–A′ of the whole optical system for this stereo imaging unitis not parallel with the parallactic direction B–B′ of the parallacticimage projected onto the image pickup device 4; the parallecticdirection B–B′ is inclined with respect to the parallactic directionA–A′. This is because the first reflecting surfaces 21L, 21R and thesecond reflecting surfaces 22L, 22R are inclined biaxially rather than asimple axis orthogonal to the same plane, so that the image of thesubject projected onto the image pickup device 4 rotates. Here, the leftand right chief rays 10L and 10R are defined by the center rays of lightbeams arriving at the centers of the left and right parallactic imagesformed on the image pickup device 4, respectively, by light incidentfrom the objective lens groups 1L and 1R on the image-formation lensgroup 3 by way of the first reflecting surfaces 21L and 21R, and thenthe second reflecting surfaces 22L and 22R.

Here, the stereo imaging optical system in this embodiment is brieflyexplained. The left and right chief rays 10L and 10R are defined asmentioned above. On the other hand, the left objective lens group 1L hasan optical axis (a center axis or axis of rotation) 11 _(1L), the rightobjective lens group 1R has an optical axis (a center axis or axis ofrotation) 11 _(1R), and the image-formation lens group 3 has one opticalaxis (a center axis or axis of rotation) 11 ₃. Given that an opticalpath at the first reflecting surfaces 21L, 21R and the second reflectingsurfaces 22L, 22R is taken apart to take the left and right opticalsystems (lens systems) as being each one lens system, the optical axis11 _(1L) of the left objective lens group 1L and the optical axis 11 ₃of the image-formation lens group 3 are lined up into one optical axis,and the optical axis 11 _(1R) of the right objective lens group 1R andthe optical axis 11 ₃ of the image-formation lens group 3 are lined upinto one optical axis. Left and right light beams from the same subjectenter the left and right objective lens groups 1L and 1R along the leftand right chief rays 10L and 10R, respectively, forming left and rightparallactic images on the lower and upper halves of the rectangularimage pickup plane of the image pickup device 4 in an inverted fashion.

It is here noted that the chief rays 10L and 10R incident on the leftand right objective lens groups 1L and 1R are not in alignment with theoptical axes 11 _(1L) and 11 _(1R) thereof; the left incident chief ray10L makes an upward angle with the left optical axis 11 _(1L) and theright incident chief ray 10R makes a downward angle with the rightoptical axis 11 _(1R). However, it is noted that in order to form theleft and right parallactic images, the chief rays 10L and 10R incidenton the left and right objective lens groups 1L and 1R make an internalangle depending on a subject distance while they are parallel with eachother or lie in much the same plane, and so the optical axes 11 _(1L)and 11 _(1R) of the left and right objective lens groups 1L and 1R aremutually twisted into 180° rotational symmetry with respect to theoptical axis 11 ₃ of the image-formation lens group 3.

On the entrance sides of the left and right objective lens groups 1L and1R, the field masks 5L and 5R are located to transmit image-formationlight beams with centers on the chief rays 10L and 10R, respectively,and limit unnecessary light. In FIG. 1( a), a relatively simple fieldmask 5L is provided to cover a substantially lower half of the leftobjective lens group 1L and a similar field mask 5R is provided to covera substantially upper half of the right objective lens group 1R, and inFIG. 1( b), an oblong, rectangular field mask 5L is provided to cover asubstantially lower half of the left objective lens group 1L therebylimiting a parallactic image formed on the image pickup device 4 to ahorizontally long, rectangular shape and a similar field mask 5R isprovided to cover a substantially upper half of the right objective lensgroup 1R thereby limiting a parallactic image formed on the image pickupdevice 4 to a horizontally long, rectangular shape.

The first reflecting surfaces 21L and 21R are provided in such a sizeand shape so as not to limit effective light beams transmitting throughthe objective lens groups 1L and 1R, and inclined at an angle of 45° inthe horizontal direction and at an angle of a few degrees toward theimage pickup device 4 in the vertical direction, so that light beamsreflected thereat are incident on the second reflecting surfaces 22L and22R. The second reflecting surfaces 22L and 22R are inclined at an angleof substantially 90° in the horizontal direction and at a minute angletoward the image pickup device in the vertical direction, so that lightbeams reflected thereat are incident on the image-formation lens group3. Referring to the second reflecting surfaces 22L and 22R as shown inFIGS. 1( a) and 1(b), they are positioned such that the upper, leftsecond reflecting surface 22L and the lower, right second reflectingsurface 22R cross each other as viewed from the vertical direction, sothat light beams coming from the left-and-right direction are polarizedin such a way as to enter the image-formation lens group 3 from thevertical direction. It is here noted that the second reflecting surfaces22L and 22R form together a stop member to create an exit pupil.

The light beams limited by the field masks 5L and 5R pass throughlow-pass filters, not shown, and then enter the image-formation lensgroup 3, thereby forming the respective parallactic images on theassociated lower and upper half areas of the image pickup device 4.Through the action of the field masks 5L and 5R, the upper and lowerparallactic images are formed on the image pickup device 4 in aparallel, separate fashion without overlapping.

It is here important that a pair of the same components be located atleft and right positions of 180° rotational symmetry about the opticalaxis 11 ₃ of the image-formation lens group 3. More specifically, a pairof the same objective lens groups 1L and 1R as well as a pair of thesame light-guide optical systems 2L and 2R should be located at left andright positions of 180° rotational symmetry about the optical axis 11 ₃of the image-formation lens group 3.

In this case, the left and right parallactic images can be picked up atwide angles of view, because the objective lens group 1L, 1R hasnegative refracting power and the image-formation lens group 3 haspositive refracting power and, hence, each of the left and right opticalsystems is of retrofocus construction.

In FIGS. 1( b) and 1(b), it is noted that reference numerals 6 and 7stand for upper and lower spaces for receiving circuit components in thestereo imaging unit.

The stereo imaging optical system in the instant stereo imaging unit arenow explained in detail.

As described above, the left and right objective lens groups 1L, 1R aswell as the left and right light-guide optical systems 2L, 2R are of thesame construction and located at positions of 180° rotational symmetryabout the optical axis 11 ₃ of the image-formation lens group 3, whilethe optical axes 11 _(1L) and 11 _(1R) of the objective lens groups 1Land 1R and the optical axis 113 of the image-formation lens group 3 arelined up into one optical axis. For this reason, the left optical systemis mainly now explained.

FIG. 2 is a taken-apart optical path diagram for the left optical systemmade up of an objective lens group 1L, a first reflecting surface 21L, asecond reflecting surface 22L and an image-formation lens group 3. Thisis a rotationally symmetric optical system with its center on an opticalaxis 11L, as shown by a dotted line. Typically, the first and secondreflecting surfaces 21L and 22L are represented by straight linesvertical to the optical axis 11L. With the optical path not taken apart,the optical system arrangement is not of rotational symmetry, becausethe first and second reflecting surfaces 21L and 22L are biaxiallyinclined about the optical axis 11L, as will be described later.

As can be seen from FIG. 2, the left objective lens group 1L is athree-lens group that is of negative power and consists of a negativemeniscus lens L₁ convex on its object side, a planoconcave negative lensL₂ and a positive meniscus lens L₃ convex on its object side; theimage-formation lens group 3 is a four-lens group that is of positivepower and consists of a double-convex positive lens L₄, a positivemeniscus lens L₅ convex on its object side, a planoconcave negative lensL₆ and a double-convex positive lens L₇; and a stop is located at aposition of the second reflecting surface 22L (the stop is formed by thecontour of the second reflecting surface 22L). The angle of view (theangle which the optical axis 11L makes with the chief ray 10L), at whichthe chief ray 10L, arriving at the center of the left parallactic imageformed on the lower half of the image pickup plane of the image pickupdevice 4 (the image plane), passes is 9.7°.

Lens data on this lens system will be tabulated in Table 1, given later.In Table 1, r₁, r₂ and so on stand for the radii of curvature of lenssurfaces (including reflecting surfaces and the image plane) as countedin order from the entrance-side surface of lens L₁, d₁, d₂ and so onindicate the spacing between adjacent lens surfaces, and ν_(d1), ν_(d2)and so on represent the Abbe numbers of lenses. In Table 1, the fullangle of view of this lens system itself is shown; however, the angle ofview of the left optical system indicated is 14° in 2.7° to 16.7° in9.7°±7° in the longitudinal (vertical) direction, and 42° in theleft-and-right (horizontal or parallactic) direction.

FIGS. 3 and 4 are a longitudinal aberration diagram and a transverseaberration diagram for this lens system of this embodiment,respectively. In these aberration diagrams, “SA”, “AS”, “DT”, “CC”, and“ω” represent spherical aberrations, astigmatisms, distortions,chromatic aberrations of magnification, and angles of view of oneoptical system, respectively.

Referring here to the coordinates with respect to the center of theimage plane (image pickup plane) of the image pickup device 4, thepositive z-axis is the normal direction to the subject (coming downthrough the image plane), the positive x-axis is the direction from theimage plane to the objective lens groups, and the y-axis is orthogonalto the z- and x-axes, giving a right-handed coordinate system.

FIG. 5 is a diagram upon projection onto the x-z plane of the leftoptical system in the stereo imaging unit according to the instantembodiment, and FIG. 6 is a similar diagram upon projection onto the y-zplane. The first and second reflecting surfaces 21L and 22L arebiaxially inclined, as already described. The inclination (rotation) ofthe first reflecting surface 21L, and the second reflecting surface 22Lis defined as the angle of rotation about the y-axis; the position ofthe normal to each reflecting surface toward the positive z-axisdirection is defined as the angle of rotation of 0°, provided that withrespect to the positive y-axis direction of the coordinate system givenfor the center of the image plane, clockwise rotation andcounterclockwise rotation are positive and negative, respectively. Thex-axis of the coordinate system given for the center of the image planeis assumed to rotate with the rotation of each reflecting surface; thatis, the x-axis is assumed to lie orthogonal to the y-axis in eachreflecting surface. With respect to the newly determined position x-axisdirection, clockwise rotation and counterclockwise rotation are definedas positive and negative, respectively.

In the instant embodiment, the angle of rotation about the y-axis of thefirst reflecting surface 21L is −44.6°, and the angle of rotation aboutthe x-axis is +8°; and the angle of rotation of the second reflectingsurface 22L about the y-axis is +45°, and the angle of rotation aboutthe x-axis is −1.2°, as shown in Table 2.

The apex positions of lens surfaces (including reflecting surfaces andthe image plane) in the coordinate system given with respect to theimage plane of the image pickup device 4 are shown in Table 3, givenlater.

FIG. 7 is a perspective view of the left optical system shown in thetaken-apart optical path diagram of FIG. 2, the diagram of FIG. 5 uponprojection onto the x-z plane, and the diagram of FIG. 6 upon projectiononto the y-z plane.

TABLE 1 (Lens Data) r₁ = 46.993 d₁ = 2 n_(d) = 1.72916 ν_(d1) = 54.68 r₂= 15.223 d₂ = 4 r₃ = ∞ d₃ = 2.26 n_(d) = 1.7725 ν_(d1) = 49.6 r₄ =22.203 d₄ = 1.1 r₅ = 31.402 d₅ = 3 n_(d) = 1.78472 ν_(d1) = 25.68 r₆ =358.141 d₆ = 17 r₇ = ∞ (1st reflecting d₇ = 49.76 surface) r₈ = ∞ (2ndreflecting d₈ = 8.24 surface) (Stop) r₉ = 18.41 d₉ = 3.13 n_(d) = 1.7725ν_(d1) = 49.6 r₁₀ = −116.92 d₁₀ = 0.1 r₁₁ = 11.528 d₁₁ = 2.7 n_(d) =1.72916 ν_(d1) = 54.68 r₁₂ = 31.126 d₁₂ = 1 r₁₃ = ∞ d₁₃ = 1.5 n_(d) =1.84666 ν_(d1) = 23.78 r₁₄ = 9.136 d₁₄ = 3.14 r₁₅ = 13.443 d₁₅ = 2.38n_(d) = 1.51633 ν_(d1) = 64.14 r₁₆ = −193.123 d₁₆ = 11.7 r₁₇ = ∞ (Imageplane) Focal length  5.0 mm F-number  1.8 Full angle 55.0° of view

TABLE 2 (Angle of inclination of reflecting surface) Angle of rotationAngle of rotation about the y-axis about the x-axis 1st reflecting−44.6° +8.0° surface 2nd reflecting +45.0° −1.2° surface

TABLE 3 (Apex position of each surface) Surface x- y- z- No. coordinatescoordinates coordinates 1 49.74 −3.47 −4.97 2 49.74 −3.13 −6.94 3 49.74−2.46 −10.88 4 49.74 −2.08 −13.11 5 49.74 −1.89 −14.19 6 49.74 −1.39−17.15 7 49.74 1.48 −33.91 8 0.00 0.00 −33.89 9 0.00 0.00 −25.65 10 0.000.00 −22.52 11 0.00 0.00 −22.42 12 0.00 0.00 −19.72 13 0.00 0.00 −18.7214 0.00 0.00 −17.22 15 0.00 0.00 −14.08 16 0.00 0.00 −11.70 17 0.00 0.000.00 (Image plane)

In Tables 1–3, dimensions are given in mm.

While the left optical system in the stereo imaging optical systemaccording to the instant embodiment has been described, it is understoodthat the right optical system holds the image-formation lens group 3 incommon, the right objective lens group 1R and the right light-guideoptical system 2R are the same in construction as the left objectivelens group 1L and the left light-guide optical system 2L, and the rightobjective lens group 1R and the right light-guide optical system 2R arelocated at mutually 180° rotationally symmetric positions about theoptical axis of the image-formation lens group 3, so that the rightparallactic image is formed on the upper half of the image pickup planeof the same image pickup device 4.

FIG. 8 is illustrative of in what relations a left parallactic image 12Land a right parallactic image 12R are projected onto the image pickupplane of an image pickup device 4. As shown, the image pickup plane ofthe image pickup device 4 is configured in such a rectangular shape asto have a long-side direction and a short-side direction, wherein thelong-side direction is clockwise rotated 13.1° toward the positivez-axis direction in association with the rotation of the left and rightparallactic images 12L and 12R, and the right parallactic image 12R andthe left parallactic image 12L are projected side by side in theshort-side direction that is substantially orthogonal to the directionof scanning by the rectangular image pickup plane of the image pickupdevice 4. Here, a chief ray 10L to enter a left optical system isincident on a position of x=0.19 mm and y=−0.82 mm on the image pickupplane, and a chief ray 10R to enter a right optical system is incidenton a position of x=−0.19 mm and y=0.82 mm symmetric with respect to thatposition. It is noted that the coordinate system used herein is the sameas already defined about the image plane (image pickup plane) of theimage pickup device 4.

As can be appreciated from the foregoing, the stereo imaging opticalsystem according to the instant embodiment is designed such thatincident light beams from the left and right objective lens groups 1Land 1R that are of the same construction and displaced in theparallactic direction are guided through the left and right light-guideoptical systems 2L and 2R of the same construction comprising tworeflecting surfaces 21L, 22R and 22L, 22R for each to an entrance pupilposition of the common image-formation lens group 3, projecting the leftand right parallactic images onto the lower and upper halves of theimage plane of the image-formation lens group 3 via the lower and upperhalves of the entrance pupil of the image-formation lens group 3. Inaddition, the objective lens groups 1L and 1R are located such that theobjective lens group 1L and the image-formation lens group 3, and theobjective lens group 1R and the image-formation lens group 3 formtogether the same coaxial optical system. To use light beams in thegiven range of upper and lower angles-of-view of equivalently oneobjective lens group to project the right and left parallactic imagesonto the same image plane in a separate fashion, it is thus requiredthat the chief rays 10L, 10R to be incident on the left and rightobjective lens groups 1L, 1R be parallel with each other or form a smallinternal angle depending on a subject distance for the purpose ofpermitting those given upper and lower angles-of-view ranges to coverthe same angle-of-view range for the subject. For this reason, theoptical path must be guided through the light-guide optical systems 2L,2R, each comprising a plurality of reflecting surfaces and located onthe way to the image-formation lens group 3, in such a way that theoptical axes 11 _(1L), 11 _(1R) of the left and right objective lensgroups 1L, 1R are mutually twisted about the 11 ₃ of the image-formationlens group 3 rather than parallel with each other. Accordingly, themutual angle difference between the optical axes 11 _(1L) and 11 _(1R)of the left and right objective lens groups 1L and 1R becomes largerthan that between the incident chief rays 10L and 10R. While theincident chief rays 10L, 10R are parallel with each other or form asmall internal angle depending on the subject distance as describedabove, the optical axis 11 _(1L) of the left objective lens group 1L andthe optical axis 11 _(1R) of the right objective lens group 1R areunparallel with, and opposite to, each other. It is here noted that theoptical axis 11 _(1L) of the left objective lens group 1L does not crossthe optical axis 11 _(1R) of the right objective lens group 1R, and theangle difference upon projection of one onto a plane including anotherhas such relations as described above.

As set forth above, the image pickup plane of the image pickup device 4is inclined in association with the rotation of the left and rightparallactic images 12L, 12R, and so the parallactic direction of theparallactic images 12L, 12R should preferably stay substantiallyparallel with scanning lines by the image pickup device 4. Usually,images gleaned through an imaging system are transferred by horizontalscanning to a frame memory in an image processor system, where they aretemporarily stored for a series of later image processing. As oneexemplary image processing, consider now matching point retrieval forparallactic image stereo matching between the parallactic images 12L and12R. It is when matching point retrieval is performed in the parallacticdirection that efficiency becomes highest, because the matching pointsin the parallactic images are found in the parallactic direction.

From the standpoint of read addressing, efficiency becomes highest whenimage information stored in the frame memory as described above is readsequentially in storage order. In other words, if horizontal scanning isperformed in the parallactic direction, it is then possible to pick upimages in the most efficient fashion for later image processing.

Next consider parallel processing for faster image processing. In thiscase, too, only simple processing is needed, because processing ofimages on horizontal scanning lines shifted by given amounts in thevertical scanning direction involves only addition of offsets to pixelreading addresses. In either case, effective processing isimplementable.

With the instant embodiment wherein the image pickup plane of the imagepickup device 4 like a CCD is inclined and the parallactic direction issubstantially parallel with scanning lines, therefore, image reading forfaster-image processing is implementable without recourse of anysquandering addressing.

Referring generally to the embodiments of FIGS. 1-8, the optical path isbent by the first reflecting surfaces 21L, 21R and the second reflectingsurfaces 22L, 22R in the light-guide optical systems 2L, 2R in such away that the image pickup plane of the common, single image pickupdevice 4 faces away from the subject side. However, it is acceptablethat the light-guide optical systems 2L, 2R are designed such that theimage pickup plane of the image pickup device 4 faces the subject side.This is now explained typically with reference to FIGS. 1( a) and 1(b).The first reflecting surface 21L on the left side bends an optical pathcoming from the left objective lens group 1L at an angle ofsubstantially 90° toward the right objective lens group 1R, and thesecond reflecting surface 22L bends the thus bent optical path at anangle of substantially 90° in a direction substantially parallel with anoptical path entering the left objective lens group 1L and in much thesame direction of that entering optical path, entering the thus bentoptical path in the common image-formation lens group 3. Likewise, thefirst reflecting surface 21R on the right side bends an optical pathcoming from the right objective lens group 1R at an angle ofsubstantially 90° toward the left objective lens group 1L, and thesecond reflecting surface 22R bends the thus bent optical path at anangle of substantially 90° in a direction substantially parallel with anoptical path entering the right objective lens group 1R and in much thesame direction of that entering optical path, entering the thus bentoptical path in the common image-formation lens group 3. In this case,too, the left and right objective lens groups 1L, 1R as well as the leftand right light-guide optical systems 2L, 2R could be of the sameconstruction and located at mutually 180° rotationally symmetricpositions about the optical axis 11 ₃ of the image-formation lens group3, and the optical axes 11 _(1L), 11 _(1R) of the left and rightobjective lens groups 1L, 1R and the optical axis 11 ₃ of theimage-formation lens group 3 could be lined up into one optical axis.

As can also be seen from FIGS. 1( a) and 1(b), the light beams incidenton the objective lens groups 1L, 1R are limited by the field masks 5L,5R. The opening in each field mask 5L, 5R is configured in asemi-circular shape substantially about each chief ray 10L, 10R or in arectangular shape that is oblong in the parallactic direction. Thoseopenings are located at positions off the optical axes 11 _(1L), 11_(1R) of the objective lens groups 1L, 1R, and effective areas throughwhich light beams incident on the objective lens groups 1L, 1R pass areeccentric with respect to the optical axes 11 _(1L), 11 _(1R), too.Therefore, when portions in the objective lens groups 1L, 1R other thanthose effective areas are trimmed off, it is preferable that trimming iscarried out such that the contour of a lens in at least either one ofthe objective lens groups 1L, 1R is in a non-rotationally symmetricshape that comes closest to the optical axes 11 _(1L), 11 _(1R) on aside, on which the chief rays 10L, 10R are not incident.

FIG. 9 is typically illustrative of the construction of a stereo imagingsystem to which one stereo imaging unit embodiment of the invention isapplied. The stereo imaging system is now explained as an onboardsystem.

That is, this stereo imaging system is made up of a distance image inputunit 100, a control unit 104, an object identification unit 105, analarm unit 106, a operating unit 107, a display unit 108, a speed sensor109, a range-finder radar 110, an illuminance sensor 111, an externalcamera 112, a GPS (global poisoning system) 113, a VICS (vehicleinformation and communications system) 114 and an externalcommunications unit 115.

Here the aforesaid distance image input unit 100 is built up of a stereoimaging unit 116 having an image pickup device 102 for phototaking asubject 400 and a stereo imaging optical system 101 mounted in front ofthat image pickup device 102, and a distance image processor 103 formeasuring a distance image 205 of the subject 400.

As is the case with generally available video cameras, digital stillcameras or the like, the stereo imaging unit 116 is optionally providedwith a phototaking stop controller (not shown), a phototaking focuscontroller (not shown), a phototaking shutter speed controller (notshown) and a sensitivity controller (not shown).

The stereo imaging optical system 101 includes a reflecting opticalsystem 211 comprising a plurality of mirror (a pair of mirrors 101A and101B). This reflecting optical system 211 is mounted in front of aforward filter group 102A in such a way that images of the subject 400incident from an objective lens group 101C and coming from differentpoints of view are formed on the image pickup device 102 through a relaylens group 101D via the filter group 102A.

A stereo image 201 phototaken at the stereo imaging unit 116, i.e., onecaptured at the image pickup device 102 is fed to the distance imageprocessor 103 as shown in FIG. 9, where it is processed into athree-dimensional distance image 205 that is in turn sent to thecontroller 104 and object identification unit 105.

It is noted that the term “distance image” used herein stands for animage having distance information in a subject's image pixel.

It is noted that reference numeral 212 in FIG. 9 is indicative of anexposure controller that is connected to the aforesaid phototaking stopcontroller, phototaking focus controller, phototaking shutter speedcontroller and sensitivity controller that the stereo-imaging unit 116has, all not shown. The exposure controller 212 is also connected to thecontroller 104 for controlling the imaging unit 116 in dependence on anexposure value calculated on the basis of brightness information fromthe image pickup device 102.

As described above, the stereo image 201 picked up at the image pickupdevice 102 is entered in the distance image processor 103. The stereoimage 201 is further entered in a frame memory 213, presenting a digitalimage 202.

The output of the frame memory 213 is entered in a rectifier 214, fromwhich a left image 203 and a right image 204 are sent out to a distancecalculator 215. The distance calculator 215 delivers a three-dimensionalimage 205 to the object identification unit 105 by way of a distanceimage output 216. The distance calculator 215 also delivers atwo-dimensional image (stereo image 201), distance image 205, etc. tothe controller 104.

It is noted that the distance image processor 103 also includes aseparate calibrator 217 that delivers a rectification parameter to therectifier 214, a distance-calculation parameter to the distancecalculator 215 and an object-identification parameter to the objectidentification unit 105.

The object identification unit 105 makes use of the enteredthree-dimensional distance image 205 to identify an object or an objectarea present therein and, delivers out the resulting object data (notshown).

Each of the components in the distance image processor 103 could beimplemented on computer software.

The controller 104 has a role in integration of image information andvehicle information. For instance, it is operable to display the resultsof processing at the distance image processor 103 on the display unit108, make an analysis of distance information obtained at the distanceimage processor 103, information from the speed sensor 103, etc. toactuate the alarm unit 106 to give an alarm, and control the operatingunit 107 to urge a driver to drive carefully. The alarm unit 106comprises a voice-warning device, a vibrator and so on. For instance,the voice-warning device produces voices from a speaker or the like, andthe vibrator forces a driver's seat to vibrate to give an alarm to thedriver.

Although how to operate the system incorporating the stereo imaging unitis not explained at great length because of having no direct relation tothe invention, it is noted that image information obtained from thestereo imaging unit 116 of the invention and the distance image inputunit 100 and vehicle information garnered from various sensors can beintegrated to display the image information on the display unit 108,actuate the alarm unit 106 to issue an alarm, and control or otherwiseoperate the operating unit 107, thereby urging the driver to drivecarefully.

For instance, as the vehicle comes too close to a subject in front, notonly a cautionary display or alarm can be shown or issued but also theoperating unit 107 (e.g., a brake) can be controlled.

Alternatively, that system can be used to read a highway centraldivision for automatic control of operation, etc.

Besides, the system of this embodiment may be used as sensors to senseother vehicles in front and in the rear, obstacles, white lines, etc.;to detect the positions and directions of the driver and a passenger(s)for prevention of dozing and looking aside while driving; and to actuatean airbag safely while judging whether those on board are children oradults and the positions and directions of their faces.

The stereo imaging unit of the invention could be applied not only to anonboard stereo imaging system but also to robots, railways, airplanes,ships, surveillance cameras, cameras for teleconferencing systems, etc.

In FIG. 9, it is noted that the mirrors 101A and 101B correspond to thefirst reflecting surfaces 21L, 21R and the second reflecting surfaces22L, 22R, respectively, in FIGS. 1–8, the objective lens group 101C tothe objective lens groups (negative lens groups) 1L, 1R, theimage-formation lens group 102A to the image-formation lens group(positive lens group) 3, and the image pickup device 102 to the imagepickup device 4.

As can be understood from the foregoing, the present invention canprovide a stereo imaging unit comprising a stereo imaging optical systemthat can afford a suitable parallax and a wide angle-of-view theretowhen used with a small-format image pickup device.

It is also possible to provide a stereo imaging unit comprising asmall-format stereo imaging optical system that has a large angle ofview in the horizontal (parallactic) direction.

Further, it is possible to provide a stereo imaging unit comprising asmall-format stereo imaging optical system that has a large angle ofview in the horizontal (parallactic) direction, wherein the number ofcomponents involved can be much more reduced.

Furthermore, it is possible to provide a stereo imaging unit comprisinga stereo imaging optical system capable of making satisfactorycorrection for aberrations as well as a stereo imaging unit comprising astereo imaging optical system capable of making efficient use of imageshaving parallaxes on an image pickup device.

1. A stereo imaging unit comprising a single image pickup device and astereo imaging optical system capable of forming on said single imagepickup device at least two parallactic images having mutual parallaxes,wherein said stereo imaging optical system comprises: a first entrancewindow and a second entrance window that have entrance surfaces locatedon a subject side and are juxtaposed in a left-and-right direction, aplurality of reflecting surfaces for guiding a light beam incident onsaid first entrance window to said single image pickup device, aplurality of reflecting surfaces for guiding a light beam incident onsaid second entrance window to said single image pickup device, a firstnegative lens group having negative refracting power on the light beamincident on said first entrance window and a first positive lens groupthat is positioned on an image side of said first negative lens groupvia a longest air space in a lens system and has positive refractingpower, and a second negative lens group having negative refracting poweron the light beam incident on said second entrance window and a secondpositive lens group that is positioned on an image side of said secondnegative lens group via a longest air space in the lens system and haspositive refracting power, with satisfaction of conditions (1), (2), (3)and (4):−10.0<f _(N1) /f _(1T)<−2.0  (1)−10.0<f _(N2) /f _(2T)<−2.0  (2)1.5<f _(P1) /f _(1T)<10  (3)1.5<f _(P2) /f _(2T)<10  (4) where f_(N1) is a focal length of saidfirst negative lens group, f_(N2) is a focal length of said secondnegative lens group, f_(P1) is a focal length of said first positivelens group, f_(P2) is a focal length of said second positive lens group,f_(1T) is a focal length of the stereo imaging optical system includingsaid first negative lens group, and f_(2T) is a focal length of thestereo imaging optical system including said second negative lens group.2. The stereo imaging unit according to claim 1, which further satisfiesconditions (5) and (6):−0.4<β_(P1)<−0.06  (5)−0.4<β_(P2)<−0.06  (6) where β_(P1) is a transverse magnification ofsaid first positive lens group, and β_(P2) is a transverse magnificationof said second positive lens group, and β_(P2) is a transversemagnification of said second positive lens group.
 3. The stereo imagingunit according to claim 1, wherein a stop member to form an exit pupilis positioned in a spacing between said first negative lens group andsaid first positive lens group, and between said second negative lensgroup and said second positive lens group.
 4. The stereo imaging unitaccording to claim 3, which satisfies conditions (7) and (8) while anoptical path is taken apart:0.03<D _(PP1) /f _(P1)<1.5  (7)0.03<D _(PP2) /f _(P2)<1.5  (8) where D_(PP1) is a distance from saidstop member to an entrance surface of said first positive lens group,and D_(PP2) is the distance from said stop member to an entrance surfaceof said second positive lens group.
 5. The stereo imaging unit accordingto claim 1, wherein while an optical path entered from each entrancewindow is taken apart, each optical system is constructed as one havinga substantially common optical axis, and a lens or a lens subgroup in atleast a part of said first positive lens group and said second positivelens group is a singe lens or a lens subgroup that is located in frontof said single image pickup device and has a common optical axis.
 6. Thestereo imaging unit according to claim 5, wherein said plurality ofreflecting surfaces are arranged such that parallactic images to beprojected on said single image pickup device are projected side by sidein a direction of juxtaposition different from that of said first andsecond entrance windows.
 7. The stereo imaging unit according to claim6, wherein an optical axis of said first negative lens group and anoptical axis of said second negative lens group are unparallel with eachother, and do not lie in the same plane.
 8. The stereo imaging unitaccording to claim 7, wherein, given that a first chief ray is definedby a center ray of a light beam that reaches a center of a parallacticimage projected onto said single image pickup device via said firstnegative lens group, said plurality of reflecting surfaces and saidfirst positive lens group and a second chief ray is defined by a centerray of a light beam that reaches a center of a parallactic imageprojected onto said single image pickup device via said second negativelens group, said plurality of reflecting surfaces and said secondpositive lens group, an angle difference between said first chief rayincident on said first negative lens group and said second chief rayincident on said second negative lens group is smaller than an angledifference between the optical axis of said first negative lens groupand the optical axis of said second negative lens group.
 9. The stereoimaging unit according to claim 6, wherein an image pickup plane of saidsingle image pickup device is configured in such a rectangular shape asto have a long-side direction and a short-side direction, and thelong-side direction of said image pickup plane is inclined with respectto a parallactic direction of said stereo imaging optical system. 10.The stereo imaging unit according to claim 6, wherein said differentdirection is substantially orthogonal to a parallactic direction of saidparallactic images.
 11. The stereo imaging unit according to claim 1,wherein an image pickup plane of said single image pickup device isconfigured in such a rectangular shape as to have a long-side directionand a short-side direction, and said single image pickup device islocated such that a parallactic image by way of said first negative lensgroup and a parallactic image by way of said second negative lens groupare projected side by side in the short-side direction of said singleimage pickup device.
 12. The stereo imaging unit according to claim 1,wherein a direction of scanning by said single image pickup device isinclined with respect to a parallactic direction of said stereo imagingoptical system.
 13. The stereo imaging unit according to claim 1,wherein a direction of scanning by said single image pickup device issubstantially parallel with a parallactic direction of the parallacticimages.
 14. The stereo imaging unit according to claim 1, wherein saidsingle image pickup device is located such that a parallactic imageformed via said first negative lens group and a parallactic image formedvia said second negative lens group are projected side by side in adirection substantially orthogonal to a direction of scanning by saidsingle image pickup device.
 15. The stereo imaging unit according toclaim 1, which further comprises field-limitation members for formingsaid at least two parallactic images on an image pickup plane of saidimage pickup device in a separate fashion.
 16. The stereo imaging unitaccording to claim 15, wherein at least one of said field-limitationmembers is said first entrance window and said second entrance window,and a field mask having a substantially rectangular opening.
 17. Thestereo imaging unit according to claim 16, said field-limitation mask islocated at a position eccentric with respect to said first negative lensgroup and said second lens group.
 18. The stereo imaging unit accordingto claim 1, wherein, given that a first chief ray is defined by a centerray of a light beam that reaches a center of a parallactic imageprojected onto said single image pickup device via said first negativelens group, said plurality of reflecting surfaces and said firstpositive lens group and a second chief ray is defined by a center ray ofa light beam that reaches a center of a parallactic image projected ontosaid single image pickup device via said second negative lens group,said plurality of reflecting surfaces and said second positive lensgroup, a contour of a lens in at least either one of said first negativelens group and said second negative lens group is in a non-rotationallysymmetric shape that comes closest to an optical axis of said lens on aside thereof, on which an associated chief ray is not incident.
 19. Astereo imaging system, comprising the stereo imaging unit according toclaim 1, an image processor that is operable in response to an imagefrom said stereo imaging unit to calculate a subject distance, producinga distance signal and a controller that is operable in response to saiddistance signal to control other device.
 20. The stereo imaging systemaccording to claim 19, wherein said other device is a display device.21. The stereo imaging system according to claim 19, wherein said otherdevice is an alarm device.
 22. The stereo imaging system according toclaim 19, wherein said other device is an operating device.
 23. A stereoimaging unit comprising a single image pickup device and a stereoimaging optical system for forming at least two parallactic imageshaving mutual parallaxes on said single image pickup device,characterized in that said stereo imaging optical system comprises: afirst objective lens group having negative refracting power, and asecond objective lens group having negative refracting power and locatedwith a spacing provided therebetween, an image-formation lens grouphaving positive refracting power and located in an optical path on animage pickup device side with respect to said first objective lens groupand said second objective lens group, a 1-1^(st) reflecting surface forreflecting an incident light beam on said first objective lens grouptoward said second objective lens group and a 1-2^(nd) reflectingsurface for reflecting a light beam from said 1-1^(st) reflectingsurface toward said image pickup device, and a 2-1^(st) reflectingsurface for an incident light beam on said second objective lens grouptoward said first objective lens group and a 2-2^(nd) reflecting surfacefor reflecting a light beam from said 2-1^(st) reflecting surface towardsaid image pickup device, wherein: said 1-2^(nd) reflecting surface andsaid 2-2^(nd) reflecting surfaces are located in such a way as toreflect light beams reflected thereat toward a subject, and said singleimage pickup device is located on a side of the light beams reflected atsaid 1-2^(nd) reflecting surface and said 2-2^(nd) reflecting surface.24. The stereo imaging unit according to claim 23, wherein saidimage-formation lens group is located just in front of said single imagepickup device.
 25. The stereo imaging unit according to claim 24,wherein said image-formation lens group receives light beams for formingsaid at least two parallactic images, and has only one optical axis. 26.The stereo imaging unit according to claim 23, wherein said 1-1^(st)reflecting surface, said 1-2^(nd) reflecting surface, said 2-1^(st)reflecting surface and said 2-2^(nd) reflecting surface are arrangedsuch that parallax images to be projected onto said single image pickupdevice are projected in a direction of juxtaposition different from thatof said first objective lens group and said second objective lens group.27. The stereo imaging unit according to claim 26, wherein, given that afirst virtual optical axis is defined by a optical axis of saidimage-formation lens group as passing through said 1-2^(nd) reflectingsurface, said 1-1^(st) reflecting surface and said first objective lensgroup upon back ray tracing and a second virtual optical axis is definedby an optical axis of said image-formation lens group as passing throughsaid 2-2^(nd) reflecting surface, said 2-1^(st) reflecting surface andsaid second objective lens group upon back ray tracing, the firstvirtual optical axis entering said first objective lens group and thesecond virtual optical axis entering said second objective lens groupare unparallel with each other, and do not lie in the same plane. 28.The stereo imaging unit according to claim 26, wherein said firstobjective lens group and said second objective lens group are eachcomprised of a lens group with a rotationally symmetric optical axis,and with an optical path taken apart, each optical axis is substantiallyin alignment with an optical axis of said image-formation lens group,and an optical axis of said first objective lens group and an opticalaxis of said second objective lens group are unparallel with each otherand do not lie in the same plane.
 29. The stereo imaging unit accordingto claim 27, wherein, given that a first chief ray is defined by acenter ray of a light beam that reaches a center of a parallactic imageprojected onto said single image pickup device via said first objectivelens group, said 1-1^(st) reflecting surface, said 1-2^(nd) reflectingsurface and said image-formation lens group and a second chief ray isdefined by a center ray of a light beam that reaches a center of aparallactic image projected onto said single image pickup device viasaid second objective lens group, said 2-1^(st) reflecting surface, said2-2^(nd) reflecting surface and said image-formation lens group, anangle difference between said first chief ray incident on said firstobjective lens group and said second chief ray incident on said secondobjective lens group is smaller than an angle difference between saidfirst virtual optical axis entering said first objective lens group andsaid second virtual optical axis entering said second objective lensgroup.
 30. The stereo imaging unit according to claim 28, wherein, giventhat a first chief ray is defined by a center ray of a light beam thatreaches a center of a parallactic image projected onto said single imagepickup device via said first objective lens group, said 1-1^(st)reflecting surface, said 1-2^(nd) reflecting surface and saidimage-formation lens group and a second chief ray is defined by a centerray of a light beam that reaches a center of a parallactic imageprojected onto said single image pickup device via said second objectivelens group, said 2-1^(st) reflecting surface, said 2-2^(nd) reflectingsurface and said image-formation lens group, an angle difference betweensaid first chief ray incident on said first objective lens group andsaid second chief ray incident on said second objective lens group issmaller than an angle difference between an optical axis of said firstobjective lens group and an optical axis of said second objective lensgroup.
 31. The stereo imaging unit according to claim 26, wherein animage pickup plane of said single image pickup device is configured insuch a rectangular shape as to have a long-side direction and ashort-side direction, and the long-side direction of said image pickupplane is inclined with respect to a parallactic direction of said stereoimaging optical system.
 32. The stereo imaging unit according to claim26, wherein said different direction is a direction substantiallyorthogonal to a parallactic direction of said parallactic images. 33.The stereo imaging unit according to claim 23, wherein an image pickupplane of said single image pickup device is configured in such arectangular shape as to have a long-side direction and a short-sidedirection, and said single image pickup device is located such that aparallactic image by way of said first objective lens group and aparallactic image by way of said second objective lens group areprojected side by side in a short-side direction of said single imagepickup device.
 34. The stereo imaging unit according to claim 23,wherein a direction of scanning by said single image pickup device isinclined with respect to a parallactic direction of said stereo imagingoptical system.
 35. The stereo imaging unit according to claim 23,wherein a direction of scanning by said single image pickup device issubstantially parallel with a parallactic direction of the parallacticimages.
 36. The stereo imaging unit according to claim 23, wherein saidsingle image pickup device is located such that a parallactic imageformed via said first objective lens group and a parallactic imageformed via said second objective lens group are projected side by sidein a direction substantially orthogonal to a direction of scanning bysaid single image pickup device.
 37. The stereo imaging unit accordingto claim 23, which satisfies conditions (11), (12), (13) and (14):−10.0<f _(T1) /f ₁<−2.0  (11)−10.0<f _(T2) /f ₂<−2.0  (12)1.5<f _(K) /f ₁<10  (13)1.5<f _(K) /f ₂<10  (14) where f_(T1) is a focal length of said firstobjective lens group, f_(T2) is a focal length of said second objectivelens group, f_(K) is a focal length of said image-formation lens group,f₁ is a focal length of the stereo imaging optical system including saidfirst objective lens group, and f₂ is a focal length of the stereoimaging optical system including said second objective lens group. 38.The stereo imaging unit according to claim 23, which further satisfiescondition (15):−0.4<β_(K)<−0.06  (15) where β_(K) is a transverse magnification of saidimage-formation lens group.
 39. The stereo imaging unit according toclaim 23, wherein a stop member for forming an exit pupil is interposedbetween said first and second objective lens groups and saidimage-formation lens group.
 40. The stereo imaging unit according toclaim 39, which satisfies condition (16) while an optical path is takenapart:0.03<D _(PK) /f _(K)<1.5  (16) where D_(PK) is a distance from said stopmember to an entrance surface of said image-formation lens group, andf_(K) is a focal length of said image-formation lens group.
 41. Thestereo imaging unit according to claim 23, which further comprisesfield-limitation members for forming said at least two parallacticimages on an image pickup plane of said image pickup device in aseparate fashion.
 42. The stereo imaging unit according to claim 41,wherein at least one of said field-limitation members is a field maskthat is located on a subject side of said objective lens groups, and hasa substantially rectangular opening.
 43. The stereo imaging unitaccording to claim 42, wherein said field mask is located at a positioneccentric with respect to optical axes of said objective lens groups.44. The stereo imaging unit according to claim 23, wherein, given that afirst chief ray is defined by a center ray of a light beam that reachesa center of a parallactic image projected onto said single image pickupdevice via said first objective lens group, said 1-1^(st) reflectingsurface, said 1-2^(nd) reflecting surface and said image-formation lensgroup and a second chief ray is defined by a center ray of a light beamthat reaches a center of a parallactic image projected onto said singleimage pickup device via said second objective lens group, said 2-1^(st)reflecting surface, said 2-2^(nd) reflecting surface and saidimage-formation lens group, a contour of a lens in at least either oneof said first objective lens group and said second objective lens groupis in a non-rotationally symmetric shape that comes closest to anoptical axis of said lens on a side thereof, on which an associatedchief ray is not incident.
 45. A stereo imaging system, comprising thestereo imaging unit according to claim 23, an image processor that isoperable in response to an image from said stereo imaging unit tocalculate a subject distance, producing a distance signal and acontroller that is operable in response to said distance signal tocontrol other device.
 46. The stereo imaging system according to claim45, wherein said other device is a display device.
 47. The stereoimaging system according to claim 45, wherein said other device is analarm device.
 48. The stereo imaging system according to claim 45,wherein said other device is an operating device.
 49. A stereo imagingunit comprising a single image pickup device and a stereo imagingoptical system for forming at least two parallactic images having mutualparallaxes on said single image pickup device, wherein said stereoimaging optical system comprises: an image-formation lens group ofpositive refracting power, which is located in front of said singleimage pickup device, receives light beams for forming said at least twoparallactic images, and has only one optical axis, a first objectivelens group and a second objective lens group, each of which has negativerefractive power and which have entrance surfaces facing a subject sideand are juxtaposed with a spacing therebetween in a parallacticdirection, a first light-guide optical system that includes a 1-1^(st)reflecting surface and a 1-2^(nd) reflecting surface for guiding a lightbeam incident from a subject thereon via said first objective lens groupto said image-formation lens group, and a second light-guide opticalsystem that includes a 2-1^(st) reflecting surface and a 2-2^(nd)reflecting surface for guiding a light beam incident from the subjectthereon via said second objective lens group to said image-formationlens group.
 50. The stereo imaging unit according to claim 49, whereinsaid stereo imaging optical system is constructed such that said atleast two parallactic images having parallaxes, to be projected ontosaid single image pickup device, are projected side by side in adirection different from a parallactic direction thereof.
 51. The stereoimaging unit according to claim 49, wherein bending of an optical pathfrom said image-formation lens group by said first light-guide opticalsystem and said second light-guide optical system upon back ray tracingis effected by reflections at only four reflecting surfaces; said1-1^(st) reflecting surface, said 1-2^(nd) reflecting surface, said2-1^(st) reflecting surface and said 2-2^(nd) reflecting surface. 52.The stereo imaging unit according to claim 49, wherein, given that afirst chief ray is defined by a center ray of a light beam that reachesa center of a parallactic image projected onto said single image pickupdevice via said first objective lens group, said first light-guideoptical system and said image-formation lens group and a second chiefray is defined by a center ray of a light beam that reaches a center ofa parallactic image projected onto said single image pickup device viasaid second objective lens group, said second light-guide optical systemand said image-formation lens group, said first objective lens group isoperable as an optical system for polarizing said first chief ray, andsaid second objective lens group is operable as an optical system forpolarizing said second chief ray.
 53. The stereo imaging unit accordingto claim 52, wherein, given that a first virtual optical axis is definedby an optical axis of said image-formation lens group as passing throughsaid first light-guide optical system and said first objective lensgroup upon back ray tracing and a second virtual optical axis is definedby an optical axis of said image-formation lens group as passing throughsaid second light-guide optical system and said second objective lensgroup upon back ray tracing, the first virtual optical axis enteringsaid first objective lens group and the second virtual optical axisentering said second objective lens group are unparallel with eachother, and do not lie in the same plane.
 54. The stereo imaging unitaccording to claim 52, wherein said first chief ray incident on saidfirst objective lens group and said second chief ray incident on saidsecond objective lens group lie in substantially the same plane.
 55. Thestereo imaging unit according to claim 49, wherein said first objectivelens group and said second objective lens group are each comprised of alens group with a rotationally symmetric optical axis; an optical axisof said first objective lens group and an optical axis of said secondobjective lens group are unparallel with each other and lie at positionsof rotational symmetry about an optical axis of said image-formationoptical lens; and with an optical path taken apart, the optical axis ofeach objective lens group is substantially in alignment with the opticalaxis of said image-formation lens group.
 56. The stereo imaging unitaccording to claim 53, wherein an angle difference between said firstchief ray incident on said first objective lens group and said secondchief ray incident on said second objective lens group is smaller thanan angle difference between said first virtual optical axis enteringsaid first objective lens group and said second virtual optical axisentering said second objective lens group.
 57. The stereo imaging unitaccording to claim 55, wherein an angle difference between the saidfirst chief ray incident on said first objective lens group and saidsecond chief ray incident on said second objective lens group is smallerthan an angle difference between an optical axis of said first objectivelens group and an optical axis of said second objective lens group. 58.The stereo imaging unit according to claim 50, wherein an image pickupplane of said single image pickup device is configured in such arectangular shape as to have a long-side direction and a short-sidedirection, and the long-side direction of said image pickup plane isinclined with respect to a parallactic direction of said stereo imagingoptical system.
 59. The stereo imaging unit according to claim 50,wherein said direction different from the direction of juxtaposition ofparallactic images on said single image pickup device is substantiallyorthogonal to the parallactic direction of said parallactic images. 60.The stereo imaging unit according to claim 49, wherein an image pickupplane of said single image pickup device is configured in such arectangular shape as to have a long-side direction and a short-sidedirection, and said single image pickup device is located such that aparallactic image by way of said first objective lens group and aparallactic image by way of said second objective lens group areprojected side by side in the short-side direction of said single imagepickup device.
 61. The stereo imaging unit according to claim 50,wherein a direction of scanning by said single image pickup device isinclined with respect to a parallactic direction of said stereo imagingoptical system.
 62. The stereo imaging unit according to claim 50,wherein a direction of scanning by said single image pickup device issubstantially parallel with a parallactic direction of the parallacticimages.
 63. The stereo imaging unit according to claim 49, wherein saidsingle image pickup device is located such that a parallactic imageformed via said first objective lens group and a parallactic imageformed via said second objective lens group are projected side by sidein a direction substantially orthogonal to a direction of scanning bysaid single image pickup device.
 64. The stereo imaging unit accordingto claim 49, which satisfies conditions (11), (12), (13) and (14):−10.0<f _(T1) /f ₁<−2.0  (11)−10.0<f _(T2) /f ₂<−2.0  (12)1.5<f _(K) /f ₁<10  (13)1.5<f _(K) /f ₂<10  (14) where f_(T1) is a focal length of said firstobjective lens group, f_(T2) is a focal length of said second objectivelens group, f_(K) is a focal length of said image-formation lens group,f₁ is a focal length of the stereo imaging optical system including saidfirst objective lens group, and f₂ is a focal length of the stereoimaging optical system including said second objective lens group. 65.The stereo imaging unit according to claim 49, which further satisfiescondition (15):−0.4<β_(K)<−0.06  (15) where β_(K) is a transverse magnification of saidimage-formation lens group.
 66. The stereo imaging unit according toclaim 49, wherein a stop member for forming an exit pupil is interposedbetween said first and second objective lens groups and saidimage-formation lens group.
 67. The stereo imaging unit according toclaim 66, which satisfies condition (16) while an optical path is takenapart:0.03<D _(PK) /f _(K)<1.5  (16) where D_(PK) is a distance from said stopmember to an entrance surface of said image-formation lens group, andf_(K) is a focal length of said image-formation lens group.
 68. Thestereo imaging unit according to claim 49, which further comprisesfield-limitation members for forming said at least two parallacticimages on an image pickup plane of said image pickup device in aseparate fashion.
 69. The stereo imaging unit according to claim 68,wherein at least one of said field-limitation members is a field maskthat is located on a subject side of said objective lens groups, and hasa substantially rectangular opening.
 70. The stereo imaging unitaccording to claim 69, wherein said field mask is located at a positioneccentric with respect to optical axes of said objective lens groups.71. The stereo imaging unit according to claim 49, wherein, given that afirst chief ray is defined by a center ray of a light beam that reachesa center of a parallactic image projected onto said single image pickupdevice via said first objective lens group, said first light-guideoptical system and said image-formation lens group and a second chiefray is defined by a center ray of a light beam that reaches a center ofa parallactic image projected onto said single image pickup device viasaid second objective lens group, said second light-guide optical systemand said image-formation lens group, a contour of a lens in at leasteither one of said first objective lens group and said second objectivelens group is in a non-rotationally symmetric shape that comes closestto an optical axis of said lens group on a side thereof on which anassociated chief ray is not incident.
 72. A stereo imaging system,comprising the stereo imaging unit according to claim 49, an imageprocessor that is operable in response to an image from said stereoimaging unit to calculate a subject distance, producing a distancesignal and a controller that is operable in response to said distancesignal to control other device.
 73. The stereo imaging system accordingto claim 72, wherein said other device is a display device.
 74. Thestereo imaging system according to claim 72, wherein said other deviceis an alarm device.
 75. The stereo imaging system according to claim 72,wherein said other device is an operating device.